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{{Redirect|Arrow paradox}}
{{redirect|Achilles and the Tortoise}}
{{short description|Set of philosophical problems}}
'''Zeno's paradoxes''' are a set of ]es devised by ] to support ]' doctrine that "all is one" and that contrary to the evidence of our senses, the belief in plurality and change is mistaken, and in particular that ] is nothing but an ].
{{Primary sources|date=March 2023}}
'''Zeno's paradoxes''' are a series of ] ] presented by the ] philosopher ] (c. 490–430 BC),<ref name=":0" /><ref name=":1" /> primarily known through the works of ], ], and later commentators like ].<ref name=":1" /> Zeno devised these paradoxes to support his teacher ]'s philosophy of ], which posits that despite our sensory experiences, reality is singular and unchanging. The paradoxes famously challenge the notions of plurality (the ] of many things), motion, space, and time by suggesting they lead to ].


Zeno's work, primarily known from ] since his ] are lost, comprises forty "paradoxes of plurality," which argue against the ] of believing in multiple existences, and several arguments against motion and change.<ref name=":1" /> Of these, only a few are definitively known today, including the renowned "Achilles Paradox", which illustrates the problematic concept of infinite divisibility in ] and ].<ref name=":0" /><ref name=":1" /> In this paradox, Zeno argues that a swift runner like ] cannot overtake a slower moving ] with a head start, because the ] between them can be infinitely subdivided, implying Achilles would require an ] number of steps to catch the tortoise.<ref name=":0" /><ref name=":1" />
Several of Zeno's eight surviving paradoxes (preserved in ]'s ''Physics'' and ]'s commentary thereon) are essentially equivalent to one another; and most of them were regarded, even in ancient times, as very easy to refute. Three of the strongest and most famous&mdash;that of ] and the ], the ] argument, and that of an arrow in flight&mdash;are given here.


These paradoxes have stirred extensive philosophical and mathematical discussion throughout ],<ref name=":0" /><ref name=":1" /> particularly regarding the nature of infinity and the continuity of space and time. Initially, ]'s interpretation, suggesting a potential rather than actual infinity, was widely accepted.<ref name=":0" /> However, modern solutions leveraging the mathematical framework of ] have provided a different perspective, highlighting Zeno's significant early insight into the complexities of infinity and continuous motion.<ref name=":0" /> Zeno's paradoxes remain a pivotal reference point in the philosophical and mathematical exploration of reality, motion, and the infinite, influencing both ancient thought and modern scientific understanding.<ref name=":0" /><ref name=":1" />
Zeno's arguments are perhaps the first examples of a method of proof called '']'' also known as proof by ]. They are also credited as a source of the ] method used by ].


== History ==
Zeno's paradoxes were a major problem for ancient and medieval ]s, who found most proposed solutions somewhat unsatisfactory. More modern solutions using ] have generally satisfied mathematicians and engineers. Many philosophers still hesitate to say that all paradoxes are completely solved, while pointing out also that attempts to deal with the paradoxes have resulted in many intellectual discoveries. Variations on the paradoxes (see ]) continue to produce at least temporary puzzlement in elucidating what, if anything, is wrong with the argument.
The origins of the paradoxes are somewhat unclear, but they are generally thought to have been developed to support ]' doctrine of ], that all of reality is one, and that ''all change is impossible'', that is, that nothing ever ] or in any other respect.<ref name=":0" /><ref name=":1" /> ], citing ], says that Zeno's teacher Parmenides was the first to introduce the paradox of Achilles and the tortoise. But in a later passage, Laërtius attributes the origin of the paradox to Zeno, explaining that Favorinus disagrees.<ref>Diogenes Laërtius, ''Lives'', 9.23 and 9.29.</ref> ] attribute the paradox to Zeno.<ref name=":0" /><ref name=":1" />


Many of these paradoxes argue that contrary to the evidence of one's senses, ] is nothing but an ].<ref name=":0" /><ref name=":1" /> In ] ] (128a–d), Zeno is characterized as taking on the project of creating these ] because other philosophers claimed paradoxes arise when considering Parmenides' view. Zeno's arguments may then be early examples of a method of proof called '']'', also known as ]. Thus Plato has Zeno say the purpose of the paradoxes "is to show that their hypothesis that existences are many, if properly followed up, leads to still more absurd results than the hypothesis that they are one."<ref>''Parmenides'' 128d</ref> Plato has ] claim that Zeno and Parmenides were essentially arguing exactly the same point.<ref>''Parmenides'' 128a–b</ref> They are also credited as a source of the ] method used by Socrates.<ref>(, Diogenes Laërtius. {{Webarchive|url=https://web.archive.org/web/20101212095647/http://classicpersuasion.org/pw/diogenes/dlzeno-eleatic.htm |date=2010-12-12 }} 25ff and VIII 57).</ref>
== The Paradoxes of motion ==
=== Achilles and the tortoise ===
''"You can never catch up."''


== Paradoxes ==
:''"In a race, the quickest runner can never overtake the slowest, since the pursuer must first reach the point whence the pursued started, so that the slower must always hold a lead." (Aristotle Physics VI:9, 239b15)''
Some of Zeno's nine surviving paradoxes (preserved in ]<ref name=aristotle> {{Webarchive|url=https://web.archive.org/web/20110106095547/http://classics.mit.edu/Aristotle/physics.html |date=2011-01-06 }} "Physics" by Aristotle translated by R. P. Hardie and R. K. Gaye</ref><ref>{{cite web|title=Greek text of "Physics" by Aristotle (refer to §4 at the top of the visible screen area)|url=http://remacle.org/bloodwolf/philosophes/Aristote/physique6gr.htm#144|archive-url=https://web.archive.org/web/20080516213308/http://remacle.org/bloodwolf/philosophes/Aristote/physique6gr.htm#144|archive-date=2008-05-16}}</ref> and ] commentary thereon) are essentially equivalent to one another. Aristotle offered a response to some of them.<ref name=aristotle/> Popular literature often misrepresents Zeno's arguments. For example, Zeno is often said to have argued that the sum of an infinite number of terms must itself be infinite–with the result that not only the time, but also the distance to be travelled, become infinite.<ref>{{cite book|last= Benson|first= Donald C.|title= The Moment of Proof : Mathematical Epiphanies|year= 1999|publisher= Oxford University Press|location= New York|isbn= 978-0195117219|page= |url= https://archive.org/details/momentofproofmat00bens|url-access= registration}}</ref> However, none of the original ancient sources has Zeno discussing the sum of any infinite series. ] has Zeno saying "it is impossible to traverse an infinite number of things in a finite time". This presents Zeno's problem not with finding the ''sum'', but rather with ''finishing'' a task with an infinite number of steps: how can one ever get from A to B, if an infinite number of (non-instantaneous) events can be identified that need to precede the arrival at B, and one cannot reach even the beginning of a "last event"?<ref name=KBrown/><ref name=FMoorcroft/><ref name=Papa-G /><ref>{{cite encyclopedia |last=Huggett |first=Nick |url=http://plato.stanford.edu/entries/paradox-zeno/#ZenInf |title=Zeno's Paradoxes: 5. Zeno's Influence on Philosophy |year=2010 |encyclopedia=] |access-date=2011-03-07 |archive-date=2022-03-01 |archive-url=https://web.archive.org/web/20220301174333/https://plato.stanford.edu/entries/paradox-zeno/#ZenInf |url-status=live }}</ref>

In the paradox of ] and the ], we imagine the Greek hero Achilles in a footrace with the plodding reptile. Because he is so fast a runner, Achilles graciously allows the tortoise a head start of a hundred feet. If we suppose that each racer starts running at some constant speed (one very fast and one very slow), then after some finite ], Achilles will have run a hundred feet, bringing him to the tortoise's starting point; during this time, the tortoise has "run" a (much shorter) distance, say one foot. It will then take Achilles some further period of time to run that distance, during which the tortoise will advance farther; and then another period of time to reach this third point, while the tortoise moves ahead. Thus, whenever Achilles reaches somewhere the tortoise has been, he still has farther to go. Therefore, Zeno says, swift Achilles can never overtake the tortoise. Thus, while common sense and common experience would hold that one runner can catch another, according to the above argument, he cannot; this is the paradox.

=== The dichotomy paradox ===
''"You cannot even start."''

:''"That which is in locomotion must arrive at the half-way stage before it arrives at the goal." (Aristotle Physics VI:9, 239b10)''

Suppose Homer wants to catch a stationary bus. Before he can get there, he must get halfway there. Before he can get halfway there, he must get a quarter of the way there. Before traveling a fourth, he must travel one-eighth; before an eighth, one-sixteenth; and so on.

<math>H-\frac{B}{8}-\frac{B}{4}---\frac{B}{2}-------B</math>


=== Paradoxes of motion ===
Three of the strongest and most famous—that of Achilles and the tortoise, the ] argument, and that of an arrow in flight—are presented in detail below.
==== Dichotomy paradox ====
]
{{ quote
| That which is in locomotion must arrive at the half-way stage before it arrives at the goal.| as recounted by ], ] VI:9, 239b10
}}
Suppose ] wishes to walk to the end of a path. Before she can get there, she must get halfway there. Before she can get halfway there, she must get a quarter of the way there. Before traveling a quarter, she must travel one-eighth; before an eighth, one-sixteenth; and so on.
<timeline>
ImageSize= width:800 height:100
PlotArea= width:720 height:55 left:65 bottom:20
AlignBars= justify
Period= from:0 till:100
TimeAxis= orientation:horizontal
ScaleMajor= unit:year increment:10 start:0
ScaleMinor= unit:year increment:1 start:0
Colors=
id:homer value:rgb(0.4,0.8,1) # light purple
PlotData=
bar:homer fontsize:L color:homer
from:0 till:100
at:50 mark:(line,red)
at:25 mark:(line,black)
at:12.5 mark:(line,black)
at:6.25 mark:(line,black)
at:3.125 mark:(line,black)
at:1.5625 mark:(line,black)
at:0.78125 mark:(line,black)
at:0.390625 mark:(line,black)
at:0.1953125 mark:(line,black)
at:0.09765625 mark:(line,black)
</timeline>
The resulting sequence can be represented as: The resulting sequence can be represented as:
:'''<math> \left\{ \cdots, \frac{1}{16}, \frac{1}{8}, \frac{1}{4}, \frac{1}{2}, 1 \right\}</math>''' :<math> \left\{ \cdots, \frac{1}{16}, \frac{1}{8}, \frac{1}{4}, \frac{1}{2}, 1 \right\}</math>
This description requires one to travel an infinite number of finite distances, which Zeno argues would take an infinite time -- which is to say, it can never be completed. This sequence also presents a second problem in that it contains no first distance to run, for any possible first distance could be divided in half, and hence would not be first after all. Hence, the trip cannot even be begun. The paradoxical conclusion then would be that travel over any finite distance can neither be completed nor begun, and so all motion must be an ].


This description requires one to complete an infinite number of tasks, which Zeno maintains is an impossibility.<ref>{{cite book|last1=Lindberg|first1=David|title=The Beginnings of Western Science|date=2007|publisher=University of Chicago Press|isbn=978-0-226-48205-7|page=33|edition=2nd}}</ref>
This argument is called the '']'' because it involves repeatedly splitting a distance into two parts. It contains some of the same elements as the ''Achilles and the Tortoise'' paradox, but with a more apparent conclusion of motionlessness. It is also known as the '''Race Course''' paradox.


This sequence also presents a second problem in that it contains no first distance to run, for any possible (]) first distance could be divided in half, and hence would not be first after all. Hence, the trip cannot even begin. The paradoxical conclusion then would be that travel over any finite distance can be neither completed nor begun, and so all motion must be an ].<ref>{{cite encyclopedia |last=Huggett |first=Nick |url=http://plato.stanford.edu/entries/paradox-zeno/#Dic |title=Zeno's Paradoxes: 3.1 The Dichotomy |year=2010 |encyclopedia=] |access-date=2011-03-07 |archive-date=2022-03-01 |archive-url=https://web.archive.org/web/20220301174333/https://plato.stanford.edu/entries/paradox-zeno/#Dic |url-status=live }}</ref>
=== The arrow paradox ===
''"You cannot even move."''


This argument is called the "]" because it involves repeatedly splitting a distance into two parts. An example with the original sense can be found in an ]. It is also known as the '''Race Course''' paradox.
:''"If everything when it occupies an equal space is at rest, and if that which is in locomotion is always occupying such a space at any moment, the flying arrow is therefore motionless." (Aristotle Physics VI:9, 239b5)''


==== Achilles and the tortoise<!--'Achilles and the Tortoise' and 'Achilles and the tortoise' redirects here--> ====
Finally, in the arrow paradox, we imagine an arrow in flight. At every moment in time, the arrow is located at a specific position. If the moment is just a single instant, then the arrow does not have time to move and is at rest during that instant. Now, during the following instances, it then must also be at rest for the same reason. The arrow is always at rest and cannot move: motion is impossible.
{{Redirect|Achilles and the Tortoise}}
{{See also|Infinity#Zeno: Achilles and the tortoise|selfref=yes}}
]


{{ quote
Whereas the first two paradoxes presented divide space, this paradox starts by dividing time - and not into segments, but into points.
| In a race, the quickest runner can never over&shy;take the slowest, since the pursuer must first reach the point whence the pursued started, so that the slower must always hold a lead.| as recounted by ], ] VI:9, 239b15
}}
In the paradox of '''Achilles and the tortoise'''<!--boldface per WP:R#PLA-->, ] is in a footrace with a tortoise. Achilles allows the tortoise a head start of 100 meters, for example. Suppose that each racer starts running at some constant speed, one faster than the other. After some finite time, Achilles will have run 100 meters, bringing him to the tortoise's starting point. During this time, the tortoise has run a much shorter distance, say 2 meters. It will then take Achilles some further time to run that distance, by which time the tortoise will have advanced farther; and then more time still to reach this third point, while the tortoise moves ahead. Thus, whenever Achilles arrives somewhere the tortoise has been, he still has some distance to go before he can even reach the tortoise. As Aristotle noted, this argument is similar to the Dichotomy.<ref>{{cite encyclopedia |last=Huggett |first=Nick |url=http://plato.stanford.edu/entries/paradox-zeno/#AchTor |title=Zeno's Paradoxes: 3.2 Achilles and the Tortoise |year=2010 |encyclopedia=] |access-date=2011-03-07 |archive-date=2022-03-01 |archive-url=https://web.archive.org/web/20220301174333/https://plato.stanford.edu/entries/paradox-zeno/#AchTor |url-status=live }}</ref> It lacks, however, the apparent conclusion of motionlessness.


== Proposed solutions == ==== Arrow paradox ====
]
{{distinct|text = ]}}


{{quote|If everything when it occupies an equal space is at rest at that instant of time, and if that which is in locomotion is always occupying such a space at any moment, the flying arrow is therefore motionless at that instant of time and at the next instant of time but if both instants of time are taken as the same instant or continuous instant of time then it is in motion.<ref>{{cite web |url=http://classics.mit.edu/Aristotle/physics.6.vi.html#752 |work=The Internet Classics Archive |title=Physics |author=Aristotle |quote=Zeno's reasoning, however, is fallacious, when he says that if everything when it occupies an equal space is at rest, and if that which is in locomotion is always occupying such a space at any moment, the flying arrow is therefore motionless. This is false, for time is not composed of indivisible moments any more than any other magnitude is composed of indivisibles. |access-date=2012-08-21 |archive-date=2008-05-15 |archive-url=https://web.archive.org/web/20080515224131/http://classics.mit.edu//Aristotle/physics.6.vi.html#752 |url-status=live }}</ref>|as recounted by ], ] VI:9, 239b5|title=|source=}}
=== Proposed solutions to the arrow paradox ===


In the arrow paradox, Zeno states that for motion to occur, an object must change the position which it occupies. He gives an example of an arrow in flight. He states that at any one (durationless) instant of time, the arrow is neither moving to where it is, nor to where it is not.<ref>{{cite book | chapter-url=http://en.wikisource.org/Lives_of_the_Eminent_Philosophers/Book_IX#Pyrrho | first=Diogenes | last=Laërtius | author-link=Diogenes Laërtius | title=Lives and Opinions of Eminent Philosophers | volume=IX | chapter=Pyrrho | at=passage 72 | year=c. 230 | isbn=1-116-71900-2 | title-link=Lives and Opinions of Eminent Philosophers | access-date=2011-03-05 | archive-date=2011-08-22 | archive-url=https://web.archive.org/web/20110822084058/http://en.wikisource.org/Lives_of_the_Eminent_Philosophers/Book_IX#Pyrrho | url-status=live }}</ref>
One objection to the arrow paradox is that the arrow paradox seems to be a play on words more than anything else. In particular, the premises state that at any instant, the arrow is at rest. However, being at rest is a relative term. One cannot judge, from observing any one instant, that the arrow is at rest. Rather, one requires other, adjacent instants to assert whether, compared to other instants, the arrow at one instant is at rest. Thus, compared to other instants, the arrow would be at a different place than it was and will be at the times before and after. Therefore, the arrow moves. A mathematical account would be as follows: in the ], as the length of a moment approaches zero, the instantaneous rate of change or velocity (which is the quotient of distance over length of the moment) does not have to approach zero. This nonzero limit is the velocity of the arrow at the instant.
It cannot move to where it is not, because no time elapses for it to move there; it cannot move to where it is, because it is already there. In other words, at every instant of time there is no motion occurring. If everything is motionless at every instant, and time is entirely composed of instants, then motion is impossible.


Whereas the first two paradoxes divide space, this paradox starts by dividing time—and not into segments, but into points.<ref name=HuggettArrow>{{cite encyclopedia |last=Huggett |first=Nick |url=http://plato.stanford.edu/entries/paradox-zeno/#Arr |title=Zeno's Paradoxes: 3.3 The Arrow |year=2010 |encyclopedia=] |access-date=2011-03-07 |archive-date=2022-03-01 |archive-url=https://web.archive.org/web/20220301174333/https://plato.stanford.edu/entries/paradox-zeno/#Arr |url-status=live }}</ref>
Another solution is that the instantaneous physical state of the arrow cannot be fully specified by its position alone: one must specify both its position and its ].


=== Other paradoxes ===
=== Proposed solutions both to Achilles & the tortoise, and to the Dichotomy ===
Aristotle gives three other paradoxes.
==== Paradox of place ====
From Aristotle:
{{quote
|If everything that exists has a place, place too will have a place, and so on '']''.<ref>Aristotle {{Webarchive|url=https://web.archive.org/web/20080509083946/http://classics.mit.edu//Aristotle/physics.4.iv.html |date=2008-05-09 }}</ref>}}


==== Paradox of the grain of millet ====
Both the paradoxes of Achilles and the tortoise and that of the dichotomy depend on dividing distances into a ] of distances that become progressively smaller, and so are subject to the same counter-arguments.
{{see also|Sorites paradox}}
Description of the paradox from the ''Routledge Dictionary of Philosophy'':
{{quote
|The argument is that a single grain of ] makes no sound upon falling, but a thousand grains make a sound. Hence a thousand nothings become something, an absurd conclusion.<ref>The Michael Proudfoot, A.R. Lace. Routledge Dictionary of Philosophy. Routledge 2009, p. 445</ref>}}


Aristotle's response:
Aristotle pointed out that as the distance decreases, the time needed to cover those distances also decreases, so that the time needed also becomes increasingly small. Such an approach to solving the paradoxes would amount to a denial that it must take an infinite amount of time to traverse an infinite sequence of distances.
{{quote
|Zeno's reasoning is false when he argues that there is no part of the millet that does not make a sound: for there is no reason why any such part should not in any length of time fail to move the air that the whole bushel moves in falling. In fact it does not of itself move even such a quantity of the air as it would move if this part were by itself: for no part even exists otherwise than potentially.<ref>Aristotle {{Webarchive|url=https://web.archive.org/web/20080511153804/http://classics.mit.edu//Aristotle/physics.7.vii.html |date=2008-05-11 }}</ref>}}


Description from Nick Huggett:
Before 212 BC, ] had developed a method to derive a finite answer for the sum of infinitely many terms that get progressively smaller. Theorems have been developed in more modern ] to achieve the same result, but with a more rigorous proof of the method. These methods allow construction of solutions stating that (under suitable conditions) if the distances are always decreasing, the time is finite.
{{quote
|This is a ] argument that one cannot trust one's sense of hearing. Aristotle's response seems to be that even inaudible sounds can add to an audible sound.<ref>Huggett, Nick, "Zeno's Paradoxes", The Stanford Encyclopedia of Philosophy (Winter 2010 Edition), Edward N. Zalta (ed.), http://plato.stanford.edu/entries/paradox-zeno/#GraMil {{Webarchive|url=https://web.archive.org/web/20220301174333/https://plato.stanford.edu/entries/paradox-zeno/#GraMil |date=2022-03-01 }}</ref>}}


==== The moving rows (or stadium) ====
====Solution using mathematical series notation====
]
These solutions have at their core ]. A general geometric ] can be written as


From Aristotle:
::<math> a\sum_{k=0}^{\infty} x^k,</math>
{{quote
|... concerning the two rows of bodies, each row being composed of an equal number of bodies of equal size, passing each other on a race-course as they proceed with equal velocity in opposite directions, the one row originally occupying the space between the goal and the middle point of the course and the other that between the middle point and the starting-post. This...involves the conclusion that half a given time is equal to double that time.<ref>Aristotle {{Webarchive|url=https://web.archive.org/web/20080515224131/http://classics.mit.edu//Aristotle/physics.6.vi.html |date=2008-05-15 }}</ref>}}


An expanded account of Zeno's arguments, as presented by Aristotle, is given in ] commentary ''On Aristotle's Physics''.<ref name=":2">{{Cite book |last1=Simplikios |title=Simplicius on Aristotle's Physics 6 |last2=Konstan |first2=David |last3=Simplikios |date=1989 |publisher=Cornell Univ. Pr |isbn=978-0-8014-2238-6 |series=Ancient commentators on Aristotle |location=Ithaca N.Y}}</ref><ref name=":1">{{Citation |last=Huggett |first=Nick |title=Zeno's Paradoxes |date=2024 |encyclopedia=The Stanford Encyclopedia of Philosophy |editor-last=Zalta |editor-first=Edward N. |url=https://plato.stanford.edu/archives/spr2024/entries/paradox-zeno/ |access-date=2024-03-25 |edition=Spring 2024 |publisher=Metaphysics Research Lab, Stanford University |editor2-last=Nodelman |editor2-first=Uri}}</ref><ref name=":0">{{Cite web |title=Zeno's Paradoxes {{!}} Internet Encyclopedia of Philosophy |url=https://iep.utm.edu/zenos-paradoxes/ |access-date=2024-03-25 |language=en-US}}</ref>
which is ] and equal to ''a''/( 1 &minus; ''x'') provided that ''|x|'' < 1 (otherwise the series ]). The paradoxes may be solved by casting them in terms of geometric series. Although the solutions effectively involve dividing up the distance to be travelled into smaller and smaller pieces, it is easier to conceive of the solution as Aristotle did, by considering the ''time'' it takes Achilles to catch up to the tortoise, and for Homer to catch the bus.


According to Angie Hobbs of The University of Sheffield, this paradox is intended to be considered together with the paradox of Achilles and the Tortoise, problematizing the concept of discrete space & time where the other problematizes the concept of infinitely divisible space & time.<ref>{{Cite web |title=Zeno's Paradoxes: The Moving Rows |url=https://digitalmedia.sheffield.ac.uk/media/Zeno%27s+ParadoxesA+The+Moving+Rows/1_e2yi73na |access-date=2024-06-28 |website=The University of Sheffield Kaltura Digital Media Hub |language=en}}</ref>
In the case of Achilles and the tortoise, suppose that the tortoise runs at a constant speed of ''v'' metres per second (ms<sup>-1</sup>) and gets a head start of distance ''d'' metres (m), and that Achilles runs at constant speed ''xv'' ms<sup>-1</sup> with ''x'' > 1. It takes Achilles time ''d''/''xv'' seconds (s) to reach the point where the tortoise started, at which time the tortoise has travelled ''d''/''x'' m. After further time ''d''/''x''<sup>2</sup>''v'' s, Achilles has another ''d''/''x'' m, and so on. Thus, the time taken for Achilles to catch up is


== Proposed solutions ==
::<math> \frac{d}{v} \sum_{k=0}^\infty \left( \frac{1}{x} \right)^k = \frac{d}{v(x-1)} \, </math> seconds.


=== In classical antiquity ===
Since this is a finite quantity, Achilles ''will'' eventually catch the tortoise.
According to ], ] said nothing upon hearing Zeno's arguments, but stood up and walked, in order to demonstrate the falsity of Zeno's conclusions.<ref name=":2" /><ref name=":1" /> To fully solve any of the paradoxes, however, one needs to show what is wrong with the argument, not just the conclusions. Throughout history several solutions have been proposed, among the earliest recorded being those of Aristotle and Archimedes.


] (384 BC–322 BC) remarked that as the distance decreases, the time needed to cover those distances also decreases, so that the time needed also becomes increasingly small.<ref>Aristotle. Physics 6.9
Similarly, for the Dichotomy assume that each of Homer's steps takes a time proportional to the distance covered by that step. Suppose that it takes time ''h'' seconds for Homer to complete the ''last'' half of the distance to the bus; then it will have taken ''h/2'' s for him to complete the second-last step, traversing the distance between one quarter and half of the way of the way. The third-last step, covering the distance between one eighth and one quarter of the way to the bus, will take ''h/4'' s, and so on. The total time taken by Homer is, summing from ''k''=0 for the last step,
</ref>{{failed verification|reason=In the section cited, Aristotle says nothing about the distance decreasing |date=October 2019}}<ref>
Aristotle's observation that the fractional times also get shorter does not guarantee, in every case, that the task can be completed. One case in which it does not hold is that in which the fractional times decrease in a ], while the distances decrease geometrically, such as: 1/2 s for 1/2 m gain, 1/3 s for next 1/4 m gain, 1/4 s for next 1/8 m gain, 1/5 s for next 1/16 m gain, 1/6 s for next 1/32 m gain, etc. In this case, the distances form a convergent series, but the times form a ], the sum of which has no limit. {{Original research inline|date=October 2020}} Archimedes developed a more explicitly mathematical approach than Aristotle.</ref>
Aristotle also distinguished "things infinite in respect of divisibility" (such as a unit of space that can be mentally divided into ever smaller units while remaining spatially the same) from things (or distances) that are infinite in extension ("with respect to their extremities").<ref>Aristotle. Physics 6.9; 6.2, 233a21-31</ref>
Aristotle's objection to the arrow paradox was that "Time is not composed of indivisible nows any more than any other magnitude is composed of indivisibles."<ref>{{cite book |author=Aristotle |title=Physics |url=http://classics.mit.edu/Aristotle/physics.6.vi.html |volume=VI |at=Part 9 verse: 239b5 |isbn=0-585-09205-2 |access-date=2008-08-11 |archive-date=2008-05-15 |archive-url=https://web.archive.org/web/20080515224131/http://classics.mit.edu//Aristotle/physics.6.vi.html |url-status=live }}</ref> ], commenting on Aristotle's objection, wrote "Instants are not parts of time, for time is not made up of instants any more than a magnitude is made of points, as we have already proved. Hence it does not follow that a thing is not in motion in a given time, just because it is not in motion in any instant of that time."<ref>Aquinas. Commentary on Aristotle's Physics, Book 6.861</ref><ref>{{Cite book |last=Kiritsis |first=Paul |title=A Critical Investigation into Precognitive Dreams |date=2020-04-01 |publisher=Cambridge Scholars Publishing |year=2020 |isbn=978-1527546332 |edition=1 |pages=19 |language=en}}</ref><ref>{{Cite web |last=Aquinas |first=Thomas |author-link=Thomas Aquinas |title=Commentary on Aristotle's Physics |url=https://aquinas.cc/la/en/~Phys.Bk6.L11 |access-date=2024-03-25 |website=aquinas.cc}}</ref>


=== In modern mathematics ===
::<math> h \sum_{k=0}^{\infty} \left( \frac{1}{2} \right)^k = 2h \,\, </math> seconds.
Some mathematicians and historians, such as ], hold that Zeno's paradoxes are simply mathematical problems, for which modern ] provides a mathematical solution.<ref name=boyer>{{cite book |last=Boyer |first=Carl |title=The History of the Calculus and Its Conceptual Development |url=https://archive.org/details/historyofcalculu0000boye |url-access=registration |year=1959 |publisher=Dover Publications |access-date=2010-02-26 |page= | quote=If the paradoxes are thus stated in the precise mathematical terminology of continuous variables (...) the seeming contradictions resolve themselves. |isbn=978-0-486-60509-8 }}</ref> Infinite processes remained theoretically troublesome in mathematics until the late 19th century. With the ] definition of ], ] and ] developed a rigorous formulation of the logic and calculus involved. These works resolved the mathematics involving infinite processes.<ref name=Lee>{{cite journal |last=Lee |first=Harold | title=Are Zeno's Paradoxes Based on a Mistake? |jstor=2251675 |year=1965 |journal= ] |volume=74 |issue=296 |publisher=Oxford University Press |pages= 563–570 |doi=10.1093/mind/LXXIV.296.563}}</ref><ref name=russell>] (1956) ''Mathematics and the metaphysicians'' in "The World of Mathematics" (ed. ]), pp 1576-1590.</ref>


Some ]s, however, say that Zeno's paradoxes and their variations (see ]) remain relevant ] problems.<ref name=KBrown/><ref name=FMoorcroft>{{cite web |first=Francis |last=Moorcroft |title=Zeno's Paradox |url=http://www.philosophers.co.uk/cafe/paradox5.htm |archive-url=https://web.archive.org/web/20100418141459/http://www.philosophers.co.uk/cafe/paradox5.htm |archive-date=2010-04-18 }}</ref><ref name=Papa-G>{{cite journal |url=http://philsci-archive.pitt.edu/2304/1/zeno_maths_review_metaphysics_alba_papa_grimaldi.pdf |first=Alba |last=Papa-Grimaldi |title=Why Mathematical Solutions of Zeno's Paradoxes Miss the Point: Zeno's One and Many Relation and Parmenides' Prohibition |journal=The Review of Metaphysics |volume=50 |year=1996 |pages=299–314 |access-date=2012-03-06 |archive-date=2012-06-09 |archive-url=https://web.archive.org/web/20120609113959/http://philsci-archive.pitt.edu/2304/1/zeno_maths_review_metaphysics_alba_papa_grimaldi.pdf |url-status=live }}</ref> While mathematics can calculate where and when the moving Achilles will overtake the Tortoise of Zeno's paradox, philosophers such as Kevin Brown<ref name=KBrown>{{cite web|first = Kevin |last = Brown |title = Zeno and the Paradox of Motion |work = Reflections on Relativity |url = http://www.mathpages.com/rr/s3-07/3-07.htm |access-date = 2010-06-06 |url-status = dead |archive-url = https://archive.today/20121205030717/http://www.mathpages.com/rr/s3-07/3-07.htm |archive-date = 2012-12-05}}</ref> and Francis Moorcroft<ref name=FMoorcroft/> hold that mathematics does not address the central point in Zeno's argument, and that solving the mathematical issues does not solve every issue the paradoxes raise. Brown concludes "Given the history of 'final resolutions', from Aristotle onwards, it's probably foolhardy to think we've reached the end. It may be that Zeno's arguments on motion, because of their simplicity and universality, will always serve as a kind of ']' onto which people can project their most fundamental phenomenological concerns (if they have any)."<ref name=KBrown/>
Once again, this is a convergent sum: although Homer must take an infinite number of steps, most of these are so short that the total ''time'' required is finite. So (provided it doesn't leave for 2''h'' seconds) Homer will catch his bus.


==== Henri Bergson ====
Note that it is also easy enough to see, in both cases, that by moving at constant speeds (and in particular not stopping after each segment) Achilles will eventually catch the moving tortoise, and Homer the stationary bus, because they will eventually have moved ''far enough''. However, the solutions that employ geometric series have the advantage that they attempt to solve the paradoxes in their own terms, by denying the apparently paradoxical conclusions.
An alternative conclusion, proposed by ] in his 1896 book '']'', is that, while the path is divisible, the motion is not.<ref>{{cite book|last=Bergson|first=Henri|title=Matière et Mémoire|trans-title=Matter and Memory|url=https://antilogicalism.com/wp-content/uploads/2017/07/matter-and-memory.pdf|author-link=Henri Bergson|date=1896|pages=77–78 of the PDF|publisher=Translation 1911 by Nancy Margaret Paul & W. Scott Palmer. George Allen and Unwin|access-date=2019-10-15|archive-date=2019-10-15|archive-url=https://web.archive.org/web/20191015184719/https://antilogicalism.com/wp-content/uploads/2017/07/matter-and-memory.pdf|url-status=live}}</ref><ref>{{Cite book |last=Massumi |first=Brian |title=Parables for the Virtual: Movement, Affect, Sensation |publisher=Duke University Press Books |year=2002 |isbn=978-0822328971 |edition=1st |location=Durham, NC |pages=5–6 |language=English}}</ref>


==== Solution using calculus notation ==== ==== Peter Lynds ====


In 2003, Peter Lynds argued that all of Zeno's motion paradoxes are resolved by the conclusion that instants in time and instantaneous magnitudes do not physically exist.<ref>{{cite web|url=http://philsci-archive.pitt.edu/1197/|title=Zeno's Paradoxes: A Timely Solution|date=January 2003|access-date=2012-07-02|archive-date=2012-08-13|archive-url=https://web.archive.org/web/20120813040121/http://philsci-archive.pitt.edu/1197/|url-status=live}}</ref><ref> Lynds, Peter. Time and Classical and Quantum Mechanics: Indeterminacy vs. Discontinuity. Foundations of Physics Letter s (Vol. 16, Issue 4, 2003). doi:10.1023/A:1025361725408</ref><ref name="Time’s Up Einstein"> {{Webarchive|url=https://web.archive.org/web/20121230100640/http://www.wired.com/wired/archive/13.06/physics.html |date=2012-12-30 }}, Josh McHugh, ], June 2005</ref> Lynds argues that an object in relative motion cannot have an instantaneous or determined relative position (for if it did, it could not be in motion), and so cannot have its motion fractionally dissected as if it does, as is assumed by the paradoxes. Nick Huggett argues that Zeno is ] when he says that objects that occupy the same space as they do at rest must be at rest.<ref name=HuggettArrow/>
* d = distance between runners
* t = time


==== Bertrand Russell ====
:<math> \lim_{d \to 0}f(d) = t </math>
Based on the work of ],<ref>{{cite book |last=Russell |first=Bertrand |date=2002 |title=Our Knowledge of the External World: As a Field for Scientific Method in Philosophy |chapter=Lecture 6. The Problem of Infinity Considered Historically |publisher=Routledge |page=169 |orig-year=First published in 1914 by The Open Court Publishing Company |isbn=0-415-09605-7}}</ref> ] offered a solution to the paradoxes, what is known as the "at-at theory of motion". It agrees that there can be no motion "during" a durationless instant, and contends that all that is required for motion is that the arrow be at one point at one time, at another point another time, and at appropriate points between those two points for intervening times. In this view motion is just change in position over time.<ref name=HuggettBook>{{ cite book |title=Space From Zeno to Einstein |first=Nick |last=Huggett |year=1999 |publisher=MIT Press |isbn=0-262-08271-3}}</ref><ref>{{cite book |url=https://books.google.com/books?id=uPRbOOv1YxUC&q=at+at+theory+of+motion+russell&pg=PA198 |title=Causality and Explanation |first=Wesley C. |last=Salmon |author-link=Wesley C. Salmon |page=198 |isbn=978-0-19-510864-4 |year=1998 |publisher=Oxford University Press |access-date=2020-11-21 |archive-date=2023-12-29 |archive-url=https://web.archive.org/web/20231229215244/https://books.google.com/books?id=uPRbOOv1YxUC&q=at+at+theory+of+motion+russell&pg=PA198#v=snippet&q=at%20at%20theory%20of%20motion%20russell&f=false |url-status=live }}</ref>


==== Hermann Weyl ====
Another proposed solution is to question one of the assumptions Zeno used in his paradoxes (particularly the Dichotomy), which is that between any two different points in space (or time), there is always another point. Without this assumption there are only a finite number of distances between two points, hence there is no infinite sequence of movements, and the paradox is resolved. According to ], the assumption that space is made of finite and discrete units is subject to a further problem, given by the "]" or "distance function problem".<ref>{{cite encyclopedia| last=Van Bendegem| first=Jean Paul| title=Finitism in Geometry| url=http://plato.stanford.edu/entries/geometry-finitism/#SomParSolProDea| encyclopedia=Stanford Encyclopedia of Philosophy| access-date=2012-01-03| date=17 March 2010| archive-date=2008-05-12| archive-url=https://web.archive.org/web/20080512012132/http://plato.stanford.edu/entries/geometry-finitism/#SomParSolProDea| url-status=live}}</ref><ref name="atomism uni of washington">{{cite web| last=Cohen| first=Marc| title=ATOMISM| url=https://www.aarweb.org/syllabus/syllabi/c/cohen/phil320/atomism.htm|work=History of Ancient Philosophy, University of Washington| access-date=2012-01-03|date=11 December 2000 |url-status=dead |archive-url=https://web.archive.org/web/20100712095732/https://www.aarweb.org/syllabus/syllabi/c/cohen/phil320/atomism.htm |archive-date=July 12, 2010}}</ref> According to this, the length of the hypotenuse of a right angled triangle in discretized space is always equal to the length of one of the two sides, in contradiction to geometry. ] has argued that the Tile Argument can be resolved, and that discretization can therefore remove the paradox.<ref name=boyer/><ref>{{cite journal |jstor=187807 |title=Discussion:Zeno's Paradoxes and the Tile Argument |first=Jean Paul |last=van Bendegem |location= Belgium |year=1987 |journal=Philosophy of Science |volume=54 |issue=2 |pages=295–302|doi=10.1086/289379|s2cid=224840314 }}</ref>


=== Applications ===
==== Quantum Zeno effect ====
{{Main article|Quantum Zeno effect}}
In 1977,<ref>{{Cite journal |bibcode=1977JMP....18..756M |last1=Sudarshan |first1=E. C. G. |author-link=E. C. G. Sudarshan |last2=Misra |first2=B. |title=The Zeno's paradox in quantum theory |journal=Journal of Mathematical Physics |volume=18 |issue=4 |pages=756–763 |year=1977 |doi=10.1063/1.523304 |osti=7342282 |url=http://repository.ias.ac.in/51139/1/211-pub.pdf |access-date=2018-04-20 |archive-date=2013-05-14 |archive-url=https://web.archive.org/web/20130514062722/http://repository.ias.ac.in/51139/1/211-pub.pdf |url-status=live }}</ref> physicists ] and B. Misra discovered that the dynamical evolution (]) of a ] can be hindered (or even inhibited) through ] of the ].<ref name="u0">{{cite journal |url=http://www.boulder.nist.gov/timefreq/general/pdf/858.pdf |author1=W.M.Itano |author2=D.J. Heinsen |author3=J.J. Bokkinger |author4=D.J. Wineland |title=Quantum Zeno effect |journal=] |volume=41 |issue=5 |pages=2295–2300 |year=1990 |doi=10.1103/PhysRevA.41.2295 |pmid=9903355 |bibcode=1990PhRvA..41.2295I |access-date=2004-07-23 |archive-url=https://web.archive.org/web/20040720153510/http://www.boulder.nist.gov/timefreq/general/pdf/858.pdf |archive-date=2004-07-20 |url-status=dead }}
</ref> This effect is usually called the "]" as it is strongly reminiscent of Zeno's arrow paradox. This effect was first theorized in 1958.<ref>{{Cite journal |last=Khalfin |first=L.A. |journal=Soviet Phys. JETP |volume=6 |page=1053 |year=1958 |bibcode = 1958JETP....6.1053K |title=Contribution to the Decay Theory of a Quasi-Stationary State }}</ref>


==== Zeno behaviour ====
=== Problem with the calculus-based solution ===
In the field of verification and design of ] and ]s, the system behaviour is called ''Zeno'' if it includes an infinite number of discrete steps in a finite amount of time.<ref name="Fishwick2007">{{cite book | editor=Paul A. Fishwick | title=Handbook of dynamic system modeling | chapter-url=https://books.google.com/books?id=cM-eFv1m3BoC&pg=SA15-PA22 | access-date=2010-03-05 | edition=hardcover | series=Chapman & Hall/CRC Computer and Information Science | date=1 June 2007 | publisher=CRC Press | location=Boca Raton, Florida, USA | isbn=978-1-58488-565-8 | pages=15–22 to 15–23 | chapter=15.6 "Pathological Behavior Classes" in chapter 15 "Hybrid Dynamic Systems: Modeling and Execution" by Pieter J. Mosterman, The Mathworks, Inc. | archive-date=2023-12-29 | archive-url=https://web.archive.org/web/20231229215249/https://books.google.com/books?id=cM-eFv1m3BoC&pg=SA15-PA22#v=onepage&q&f=false | url-status=live }}</ref> Some ] techniques exclude these behaviours from analysis, if they are not equivalent to non-Zeno behaviour.<ref>{{cite journal |last=Lamport |first=Leslie |author-link=Leslie Lamport |year=2002 |title=Specifying Systems |journal=Microsoft Research |publisher=Addison-Wesley |isbn=0-321-14306-X |url=http://research.microsoft.com/en-us/um/people/lamport/tla/book-02-08-08.pdf |page=128 |access-date=2010-03-06 |archive-date=2010-11-16 |archive-url=https://web.archive.org/web/20101116164613/http://research.microsoft.com/en-us/um/people/lamport/tla/book-02-08-08.pdf |url-status=live }}</ref><ref>{{cite journal |last1=Zhang |first1=Jun |last2=Johansson| first2=Karl | first3=John |last3=Lygeros |first4=Shankar |last4=Sastry |title=Zeno hybrid systems | journal=International Journal for Robust and Nonlinear Control |year=2001 |access-date=2010-02-28 |doi=10.1002/rnc.592 |volume=11 |issue=5 |page=435 |s2cid=2057416 |url=http://aphrodite.s3.kth.se/~kallej/papers/zeno_ijnrc01.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110811144122/http://aphrodite.s3.kth.se/~kallej/papers/zeno_ijnrc01.pdf |archive-date=August 11, 2011}}</ref> In ] these behaviours will also often be excluded from system models, since they cannot be implemented with a digital controller.<ref>{{cite book|last2=Henzinger |first2=Thomas |last1=Franck |first1=Cassez |first3=Jean-Francois |last3=Raskin |url=http://mtc.epfl.ch/~tah/Publications/a_comparison_of_control_problems_for_timed_and_hybrid_systems.html |title=A Comparison of Control Problems for Timed and Hybrid Systems |year=2002 |access-date=2010-03-02 |url-status=dead |archive-url=https://web.archive.org/web/20080528193234/http://mtc.epfl.ch/~tah/Publications/a_comparison_of_control_problems_for_timed_and_hybrid_systems.html |archive-date=May 28, 2008 }}</ref>


== Similar paradoxes ==
A problem with using calculus to try and solve Zeno's paradoxes is that this only addresses the ''geometry'' of the situation, and not its ''dynamics''. What is at the core of Zeno's paradoxes is the idea that one cannot finish the act of sequentially going through an infinite sequence, and while calculus shows that the sum of an infinite number of terms can be finite, calculus does not explain how one is able to finish going through an infinite number of points, if one has to go through these points one by one. Indeed, saying that there are an infinite number of points or intervals within some finite interval is of course the ''very assumption'' in the Achilles and Dichotomy Paradoxes, and it is this assumption regarding the geometry of the situation that leads to a paradox regarding its dynamics.
=== School of Names ===
]
Roughly contemporaneously during the ] (475–221 BCE), ] philosophers from the ], a school of thought similarly concerned with logic and dialectics, developed paradoxes similar to those of Zeno. The works of the School of Names have largely been lost, with the exception of portions of the ]. The second of the Ten Theses of ] suggests knowledge of infinitesimals:''That which has no thickness cannot be piled up; yet it is a thousand li in dimension.'' Among the many puzzles of his recorded in the ] is one very similar to Zeno's Dichotomy: {{Quote|quote=<poem>"If from a stick a foot long you every day take the half of it, in a myriad ages it will not be exhausted."</poem>|source=''Zhuangzi'', chapter 33 (Legge translation)<ref>{{Cite book |title=Sacred Books of the East |publisher=] |year=1891 |editor-last=Müller |editor-first=Max |volume=40 |translator-last=Legge |translator-first=James |chapter=The Writings of Kwang Tse}}</ref>}} ] appears to propose a solution to this paradox by arguing that in moving across a measured length, the distance is not covered in successive fractions of the length, but in one stage. Due to the lack of surviving works from the School of Names, most of the other paradoxes listed are difficult to interpret.<ref>{{Cite web |title=School of Names > Miscellaneous Paradoxes (Stanford Encyclopedia of Philosophy) |url=https://plato.stanford.edu/entries/school-names/paradoxes.html |access-date=2020-01-30 |website=plato.stanford.edu |archive-date=2016-12-11 |archive-url=https://web.archive.org/web/20161211103807/https://plato.stanford.edu/entries/school-names/paradoxes.html |url-status=live }}</ref>


=== Lewis Carroll's "What the Tortoise Said to Achilles" ===
It should also be noted that calculus-based solutions that are offered often object to the claim that "it must take an infinite amount of time to traverse an infinite sequence of distances". However, Zeno's paradox doesn't contemplate the time it would take for Achilles to catch the Tortoise; it simply points out that in order for Achilles to catch up with the Tortoise, Achilles must first perform an infinite number of acts, which seems to be impossible in and of itself: time has nothing to do with it. Thus, calculus-based solutions to Zeno's paradoxes often make the paradox into a ].
{{Main article|What the Tortoise Said to Achilles}}
"What the Tortoise Said to Achilles",<ref>{{Cite journal|last=Carroll|first=Lewis|title=What the Tortoise Said to Achilles|date=1895-04-01|url=https://academic.oup.com/mind/article/IV/14/278/1046872|journal=Mind|language=en|volume=IV|issue=14|pages=278–280|doi=10.1093/mind/IV.14.278|issn=0026-4423|access-date=2020-07-20|archive-date=2020-07-20|archive-url=https://web.archive.org/web/20200720045852/https://academic.oup.com/mind/article/IV/14/278/1046872|url-status=live}}</ref> written in 1895 by ], describes a paradoxical infinite regress argument in the realm of pure logic. It uses Achilles and the Tortoise as characters in a clear reference to Zeno's paradox of Achilles.<ref>{{Cite book |last=Tsilipakos |first=Leonidas |title=Clarity and confusion in social theory: taking concepts seriously |date=2021 |publisher=Routledge |isbn=978-1-032-09883-8 |series=Philosophy and method in the social sciences |location=Abingdon New York (N.Y.) |pages=48}}</ref>


== See also ==
Some people, using the notion of a ], argue that an infinite number of things can be completed in sequence as follows: do the first thing in 1 second, the second in half a second, the third in a quarter, etc. Again from a calculus point of view, this would seem to mean that after 2 seconds an infinite number of things would have been done. However, there would seem to have to be a problem with this, as getting to the end of an endless series would seem to be a contradiction in terms. Indeed, the problem with this argument is exactly the same as before. Since this line of reasoning assumes that there is a one-to-one mapping between the acts and the points (or intervals) in time during which that act takes place, one would need to go through (and finish) an infinite number of points in time in order to complete the infinite number of acts to be performed. But, once again, such an infinite sequence can, by its very infinite nature, not be finished. Indeed, following Zeno's logic as we just did now, we have created yet another version of his paradox: not only does motion through space seem to be impossible, but the flow of time itself seems to be impossible as well.
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]


== Notes ==
In short, trying to use calculus to resolve the paradox simply ''reaffirms'' the idea that space and time are infinitely divisible, and thus still suffers from the basic question as to how one can possibly ''reach the end of an endless series''.
{{Reflist|30em}}

=== Are space and time infinitely divisible? ===

Given the above analyses, a rather straightforward solution to some of the paradoxes is to deny that space and time are infinitely divisible. Just because our number system enables us to give a number between any two numbers, it does not necessarily follow that there is a point in space between any two different points in space, and the same goes for time. Indeed, physicists talk about ] and ] as the smallest meaningful, measurable units of space and of time, thus making measurements of both time and space ''discrete'' rather than ''continuous''. Of course, whether or not space and time are measurable with infinite precision is ultimately irrelevant to the paradoxes and their resolution: what we as humans can know about the world is a different matter from what is or is not true or possible in the world. That is, quantum mechanics may prevent us from making infinitely precise measurements, but if time and space are continuous, the paradoxes still apply. However, if time and space are discrete, one avoids obtaining the infinite series that underlies most of the paradoxes.

=== Does motion involve a sequence of points? ===

Some people, including ], have proposed an alternative solution. Lynds posits that the paradoxes arise because people have wrongly assumed that an object in motion has a determined relative position at any instant in time, thus rendering the body's motion static at that instant and enabling the impossible situation of the paradoxes to be derived. Lynds asserts that the correct resolution of the paradox lies in the realisation of the absence of an instant in time underlying a body's motion, and that regardless of how small the time interval, it is still always moving and its position constantly changing, so can never be determined at a time. Consequently, a body cannot be thought of as having a determined position at a particular instant in time while in motion, nor be fractionally dissected as such, as is assumed in the paradoxes (and their historically accepted solutions).

=== Conceptual approaches ===

A final approach is to deny that ''time'' and ''space'' are ] entities. That is, maybe we should give up on our ]nic view of reality, and say that ''time'' and ''space'' are simply conceptual constructs humans use to measure change, that the terms (space and time), though nouns, do not ] to any entities nor containers for entities, and that no thing is being divided up when one talks about "segments" of space or "points" in time. Thus, compatible with the last two approaches, one denies that our conceptual account of motion as point-by-point movement through continuous space-time needs to match exactly with anything in the real world.

=== The notion of different orders of infinity ===

The dichotomy paradox merely makes the point that the points on a continuum cannot be counted - from any point, there is no ''next'' point to proceed to. This is uncontroversial.

== Status of the paradoxes today ==
Mathematicians thought they had done away with Zeno's paradoxes with the invention of the ] and methods of handling infinite ]s by ] and ] in the ], and then again when certain problems with their methods were resolved by the reformulation of the calculus and infinite series methods in the 19th century. Many philosophers, and certainly engineers, generally went along with the mathematical results.

Nevertheless, Zeno's paradoxes are still hotly debated by philosophers in academic circles. Infinite processes have remained theoretically troublesome. ], a Dutch mathematician of the 19th and 20th century, and founder of the ] school, was the most prominent of those who rejected arguments, including proofs, involving infinities. In this he followed ], an earlier 19th century mathematician. It would be incorrect to say that a rigorous formulation of the calculus (as the ] version of ] and ] in the 19th century or the equivalent and equally rigorous differential/infinitesimal version by ] in the 20th) has resolved forever all problems involving infinities, including Zeno's.

As a practical matter, however, no engineer has been concerned about them since knowledge of the calculus became common at engineering schools. In ordinary life, very few people have ever been much concerned.

==Two other paradoxes as given by Aristotle==

''Paradox of Place:''

:"… if everything that exists has a place, place too will have a place, and so on '']''". (Aristotle ''Physics'' IV:1, 209a25)

''Paradox of the Grain of Millet:''

:"… there is no part of the millet that does not make a sound: for there is no reason why any such part should not in any length of time fail to move the air that the whole bushel moves in falling. In fact it does not of itself move even such a quantity of the air as it would move if this part were by itself: for no part even exists otherwise than potentially." (Aristotle ''Physics'' VII:5, 250a20)

For an expanded account of Zeno's arguments as presented by Aristotle, see: Simplicius' commentary ''On Aristotle's Physics''.

== The quantum Zeno effect ==
In recent time, physicists studying quantum mechanics have noticed that the dynamical evolution (motion) of a quantum system can be hindered (or even inhibited) through observation of the system. This effect is usually called the ] as it is strongly reminiscent of Zeno's arrow paradox.

== See also==
{{wikiquotepar|Aristotle}}
*]
*]


== References == == References ==
{{refbegin}}
* R.M. Sainsbury, ''Paradoxes'', Second Ed (Cambridge UP, 2003)
* ], ], M. Schofield (1984) ''The Presocratic Philosophers: A Critical History with a Selection of Texts, 2nd ed.'' ]. {{isbn|0-521-27455-9}}.
* {{cite encyclopedia |encyclopedia=] |title=Zeno's Paradoxes |url=http://plato.stanford.edu/entries/paradox-zeno/ |first=Nick |last=Huggett |year=2010 |access-date=2011-03-07 |archive-date=2022-03-01 |archive-url=https://web.archive.org/web/20220301174333/https://plato.stanford.edu/entries/paradox-zeno/ |url-status=live }}
* ] (1926) ''Plato: Cratylus. Parmenides. Greater Hippias. Lesser Hippias'', H. N. Fowler (Translator), ]. {{isbn|0-674-99185-0}}.
* Sainsbury, R.M. (2003) ''Paradoxes'', 2nd ed. Cambridge University Press. {{isbn|0-521-48347-6}}.
* {{cite book |last=Skyrms |first=Brian |authorlink=Brian Skyrms |chapter=Zeno's Paradox of Measure |title=Physics, Philosophy, and Psychoanalysis |editor-first=R. S. |editor-last=Cohen |editor2-first=L. |editor2-last=Laudan |editor2-link=Larry Laudan |location=Dordrecht |publisher=Reidel |year=1983 |isbn=90-277-1533-5 |pages=223–254 }}
{{refend}}


== External links == == External links ==
{{Wikisource|Catholic Encyclopedia (1913)/Zeno of Elea|Zeno of Elea}}
* http://www.mathacademy.com/pr/prime/articles/zeno_tort/index.asp
* * Dowden, Bradley. "." Entry in the ].
* {{springer|title=Antinomy|id=p/a012710}}
*
* , Ludwig-Maximilians-Universität München
*: 10<sup>&minus;16</sup> seconds.
* Silagadze, Z. K. ","
*
* '''' by Jon McLoone, ].
*
*
* {{cite encyclopedia |url=http://plato.stanford.edu/entries/zeno-elea/ |encyclopedia=Stanford Encyclopedia of Philosophy |title=Zeno of Elea |first=John |last=Palmer |year=2008}}
* {{PlanetMath attribution|id=5538|title=Zeno's paradox}}
* {{cite web|last=Grime|first=James|title=Zeno's Paradox|url=http://www.numberphile.com/videos/zeno_paradox.html|work=Numberphile|publisher=]|access-date=2013-04-13|archive-date=2018-10-03|archive-url=https://web.archive.org/web/20181003050912/http://www.numberphile.com/videos/zeno_paradox.html|url-status=dead}}


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Latest revision as of 08:24, 15 January 2025

"Arrow paradox" redirects here. For other uses, see Arrow paradox (disambiguation). Set of philosophical problems
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Zeno's paradoxes are a series of philosophical arguments presented by the ancient Greek philosopher Zeno of Elea (c. 490–430 BC), primarily known through the works of Plato, Aristotle, and later commentators like Simplicius of Cilicia. Zeno devised these paradoxes to support his teacher Parmenides's philosophy of monism, which posits that despite our sensory experiences, reality is singular and unchanging. The paradoxes famously challenge the notions of plurality (the existence of many things), motion, space, and time by suggesting they lead to logical contradictions.

Zeno's work, primarily known from second-hand accounts since his original texts are lost, comprises forty "paradoxes of plurality," which argue against the coherence of believing in multiple existences, and several arguments against motion and change. Of these, only a few are definitively known today, including the renowned "Achilles Paradox", which illustrates the problematic concept of infinite divisibility in space and time. In this paradox, Zeno argues that a swift runner like Achilles cannot overtake a slower moving tortoise with a head start, because the distance between them can be infinitely subdivided, implying Achilles would require an infinite number of steps to catch the tortoise.

These paradoxes have stirred extensive philosophical and mathematical discussion throughout history, particularly regarding the nature of infinity and the continuity of space and time. Initially, Aristotle's interpretation, suggesting a potential rather than actual infinity, was widely accepted. However, modern solutions leveraging the mathematical framework of calculus have provided a different perspective, highlighting Zeno's significant early insight into the complexities of infinity and continuous motion. Zeno's paradoxes remain a pivotal reference point in the philosophical and mathematical exploration of reality, motion, and the infinite, influencing both ancient thought and modern scientific understanding.

History

The origins of the paradoxes are somewhat unclear, but they are generally thought to have been developed to support Parmenides' doctrine of monism, that all of reality is one, and that all change is impossible, that is, that nothing ever changes in location or in any other respect. Diogenes Laërtius, citing Favorinus, says that Zeno's teacher Parmenides was the first to introduce the paradox of Achilles and the tortoise. But in a later passage, Laërtius attributes the origin of the paradox to Zeno, explaining that Favorinus disagrees. Modern academics attribute the paradox to Zeno.

Many of these paradoxes argue that contrary to the evidence of one's senses, motion is nothing but an illusion. In Plato's Parmenides (128a–d), Zeno is characterized as taking on the project of creating these paradoxes because other philosophers claimed paradoxes arise when considering Parmenides' view. Zeno's arguments may then be early examples of a method of proof called reductio ad absurdum, also known as proof by contradiction. Thus Plato has Zeno say the purpose of the paradoxes "is to show that their hypothesis that existences are many, if properly followed up, leads to still more absurd results than the hypothesis that they are one." Plato has Socrates claim that Zeno and Parmenides were essentially arguing exactly the same point. They are also credited as a source of the dialectic method used by Socrates.

Paradoxes

Some of Zeno's nine surviving paradoxes (preserved in Aristotle's Physics and Simplicius's commentary thereon) are essentially equivalent to one another. Aristotle offered a response to some of them. Popular literature often misrepresents Zeno's arguments. For example, Zeno is often said to have argued that the sum of an infinite number of terms must itself be infinite–with the result that not only the time, but also the distance to be travelled, become infinite. However, none of the original ancient sources has Zeno discussing the sum of any infinite series. Simplicius has Zeno saying "it is impossible to traverse an infinite number of things in a finite time". This presents Zeno's problem not with finding the sum, but rather with finishing a task with an infinite number of steps: how can one ever get from A to B, if an infinite number of (non-instantaneous) events can be identified that need to precede the arrival at B, and one cannot reach even the beginning of a "last event"?

Paradoxes of motion

Three of the strongest and most famous—that of Achilles and the tortoise, the Dichotomy argument, and that of an arrow in flight—are presented in detail below.

Dichotomy paradox

The dichotomy

That which is in locomotion must arrive at the half-way stage before it arrives at the goal.

— as recounted by Aristotle, Physics VI:9, 239b10

Suppose Atalanta wishes to walk to the end of a path. Before she can get there, she must get halfway there. Before she can get halfway there, she must get a quarter of the way there. Before traveling a quarter, she must travel one-eighth; before an eighth, one-sixteenth; and so on.

The resulting sequence can be represented as:

{ , 1 16 , 1 8 , 1 4 , 1 2 , 1 } {\displaystyle \left\{\cdots ,{\frac {1}{16}},{\frac {1}{8}},{\frac {1}{4}},{\frac {1}{2}},1\right\}}

This description requires one to complete an infinite number of tasks, which Zeno maintains is an impossibility.

This sequence also presents a second problem in that it contains no first distance to run, for any possible (finite) first distance could be divided in half, and hence would not be first after all. Hence, the trip cannot even begin. The paradoxical conclusion then would be that travel over any finite distance can be neither completed nor begun, and so all motion must be an illusion.

This argument is called the "Dichotomy" because it involves repeatedly splitting a distance into two parts. An example with the original sense can be found in an asymptote. It is also known as the Race Course paradox.

Achilles and the tortoise

"Achilles and the Tortoise" redirects here. For other uses, see Achilles and the Tortoise (disambiguation). See also: Infinity § Zeno: Achilles and the tortoise
Achilles and the tortoise

In a race, the quickest runner can never over­take the slowest, since the pursuer must first reach the point whence the pursued started, so that the slower must always hold a lead.

— as recounted by Aristotle, Physics VI:9, 239b15

In the paradox of Achilles and the tortoise, Achilles is in a footrace with a tortoise. Achilles allows the tortoise a head start of 100 meters, for example. Suppose that each racer starts running at some constant speed, one faster than the other. After some finite time, Achilles will have run 100 meters, bringing him to the tortoise's starting point. During this time, the tortoise has run a much shorter distance, say 2 meters. It will then take Achilles some further time to run that distance, by which time the tortoise will have advanced farther; and then more time still to reach this third point, while the tortoise moves ahead. Thus, whenever Achilles arrives somewhere the tortoise has been, he still has some distance to go before he can even reach the tortoise. As Aristotle noted, this argument is similar to the Dichotomy. It lacks, however, the apparent conclusion of motionlessness.

Arrow paradox

The arrow
Not to be confused with other paradoxes of the same name.

If everything when it occupies an equal space is at rest at that instant of time, and if that which is in locomotion is always occupying such a space at any moment, the flying arrow is therefore motionless at that instant of time and at the next instant of time but if both instants of time are taken as the same instant or continuous instant of time then it is in motion.

— as recounted by Aristotle, Physics VI:9, 239b5

In the arrow paradox, Zeno states that for motion to occur, an object must change the position which it occupies. He gives an example of an arrow in flight. He states that at any one (durationless) instant of time, the arrow is neither moving to where it is, nor to where it is not. It cannot move to where it is not, because no time elapses for it to move there; it cannot move to where it is, because it is already there. In other words, at every instant of time there is no motion occurring. If everything is motionless at every instant, and time is entirely composed of instants, then motion is impossible.

Whereas the first two paradoxes divide space, this paradox starts by dividing time—and not into segments, but into points.

Other paradoxes

Aristotle gives three other paradoxes.

Paradox of place

From Aristotle:

If everything that exists has a place, place too will have a place, and so on ad infinitum.

Paradox of the grain of millet

See also: Sorites paradox

Description of the paradox from the Routledge Dictionary of Philosophy:

The argument is that a single grain of millet makes no sound upon falling, but a thousand grains make a sound. Hence a thousand nothings become something, an absurd conclusion.

Aristotle's response:

Zeno's reasoning is false when he argues that there is no part of the millet that does not make a sound: for there is no reason why any such part should not in any length of time fail to move the air that the whole bushel moves in falling. In fact it does not of itself move even such a quantity of the air as it would move if this part were by itself: for no part even exists otherwise than potentially.

Description from Nick Huggett:

This is a Parmenidean argument that one cannot trust one's sense of hearing. Aristotle's response seems to be that even inaudible sounds can add to an audible sound.

The moving rows (or stadium)

The moving rows

From Aristotle:

... concerning the two rows of bodies, each row being composed of an equal number of bodies of equal size, passing each other on a race-course as they proceed with equal velocity in opposite directions, the one row originally occupying the space between the goal and the middle point of the course and the other that between the middle point and the starting-post. This...involves the conclusion that half a given time is equal to double that time.

An expanded account of Zeno's arguments, as presented by Aristotle, is given in Simplicius's commentary On Aristotle's Physics.

According to Angie Hobbs of The University of Sheffield, this paradox is intended to be considered together with the paradox of Achilles and the Tortoise, problematizing the concept of discrete space & time where the other problematizes the concept of infinitely divisible space & time.

Proposed solutions

In classical antiquity

According to Simplicius, Diogenes the Cynic said nothing upon hearing Zeno's arguments, but stood up and walked, in order to demonstrate the falsity of Zeno's conclusions. To fully solve any of the paradoxes, however, one needs to show what is wrong with the argument, not just the conclusions. Throughout history several solutions have been proposed, among the earliest recorded being those of Aristotle and Archimedes.

Aristotle (384 BC–322 BC) remarked that as the distance decreases, the time needed to cover those distances also decreases, so that the time needed also becomes increasingly small. Aristotle also distinguished "things infinite in respect of divisibility" (such as a unit of space that can be mentally divided into ever smaller units while remaining spatially the same) from things (or distances) that are infinite in extension ("with respect to their extremities"). Aristotle's objection to the arrow paradox was that "Time is not composed of indivisible nows any more than any other magnitude is composed of indivisibles." Thomas Aquinas, commenting on Aristotle's objection, wrote "Instants are not parts of time, for time is not made up of instants any more than a magnitude is made of points, as we have already proved. Hence it does not follow that a thing is not in motion in a given time, just because it is not in motion in any instant of that time."

In modern mathematics

Some mathematicians and historians, such as Carl Boyer, hold that Zeno's paradoxes are simply mathematical problems, for which modern calculus provides a mathematical solution. Infinite processes remained theoretically troublesome in mathematics until the late 19th century. With the epsilon-delta definition of limit, Weierstrass and Cauchy developed a rigorous formulation of the logic and calculus involved. These works resolved the mathematics involving infinite processes.

Some philosophers, however, say that Zeno's paradoxes and their variations (see Thomson's lamp) remain relevant metaphysical problems. While mathematics can calculate where and when the moving Achilles will overtake the Tortoise of Zeno's paradox, philosophers such as Kevin Brown and Francis Moorcroft hold that mathematics does not address the central point in Zeno's argument, and that solving the mathematical issues does not solve every issue the paradoxes raise. Brown concludes "Given the history of 'final resolutions', from Aristotle onwards, it's probably foolhardy to think we've reached the end. It may be that Zeno's arguments on motion, because of their simplicity and universality, will always serve as a kind of 'Rorschach image' onto which people can project their most fundamental phenomenological concerns (if they have any)."

Henri Bergson

An alternative conclusion, proposed by Henri Bergson in his 1896 book Matter and Memory, is that, while the path is divisible, the motion is not.

Peter Lynds

In 2003, Peter Lynds argued that all of Zeno's motion paradoxes are resolved by the conclusion that instants in time and instantaneous magnitudes do not physically exist. Lynds argues that an object in relative motion cannot have an instantaneous or determined relative position (for if it did, it could not be in motion), and so cannot have its motion fractionally dissected as if it does, as is assumed by the paradoxes. Nick Huggett argues that Zeno is assuming the conclusion when he says that objects that occupy the same space as they do at rest must be at rest.

Bertrand Russell

Based on the work of Georg Cantor, Bertrand Russell offered a solution to the paradoxes, what is known as the "at-at theory of motion". It agrees that there can be no motion "during" a durationless instant, and contends that all that is required for motion is that the arrow be at one point at one time, at another point another time, and at appropriate points between those two points for intervening times. In this view motion is just change in position over time.

Hermann Weyl

Another proposed solution is to question one of the assumptions Zeno used in his paradoxes (particularly the Dichotomy), which is that between any two different points in space (or time), there is always another point. Without this assumption there are only a finite number of distances between two points, hence there is no infinite sequence of movements, and the paradox is resolved. According to Hermann Weyl, the assumption that space is made of finite and discrete units is subject to a further problem, given by the "tile argument" or "distance function problem". According to this, the length of the hypotenuse of a right angled triangle in discretized space is always equal to the length of one of the two sides, in contradiction to geometry. Jean Paul Van Bendegem has argued that the Tile Argument can be resolved, and that discretization can therefore remove the paradox.

Applications

Quantum Zeno effect

Main article: Quantum Zeno effect

In 1977, physicists E. C. George Sudarshan and B. Misra discovered that the dynamical evolution (motion) of a quantum system can be hindered (or even inhibited) through observation of the system. This effect is usually called the "Quantum Zeno effect" as it is strongly reminiscent of Zeno's arrow paradox. This effect was first theorized in 1958.

Zeno behaviour

In the field of verification and design of timed and hybrid systems, the system behaviour is called Zeno if it includes an infinite number of discrete steps in a finite amount of time. Some formal verification techniques exclude these behaviours from analysis, if they are not equivalent to non-Zeno behaviour. In systems design these behaviours will also often be excluded from system models, since they cannot be implemented with a digital controller.

Similar paradoxes

School of Names

Diagram of Hui Shi's stick paradox

Roughly contemporaneously during the Warring States period (475–221 BCE), ancient Chinese philosophers from the School of Names, a school of thought similarly concerned with logic and dialectics, developed paradoxes similar to those of Zeno. The works of the School of Names have largely been lost, with the exception of portions of the Gongsun Longzi. The second of the Ten Theses of Hui Shi suggests knowledge of infinitesimals:That which has no thickness cannot be piled up; yet it is a thousand li in dimension. Among the many puzzles of his recorded in the Zhuangzi is one very similar to Zeno's Dichotomy:

"If from a stick a foot long you every day take the half of it, in a myriad ages it will not be exhausted."

— Zhuangzi, chapter 33 (Legge translation)

The Mohist canon appears to propose a solution to this paradox by arguing that in moving across a measured length, the distance is not covered in successive fractions of the length, but in one stage. Due to the lack of surviving works from the School of Names, most of the other paradoxes listed are difficult to interpret.

Lewis Carroll's "What the Tortoise Said to Achilles"

Main article: What the Tortoise Said to Achilles

"What the Tortoise Said to Achilles", written in 1895 by Lewis Carroll, describes a paradoxical infinite regress argument in the realm of pure logic. It uses Achilles and the Tortoise as characters in a clear reference to Zeno's paradox of Achilles.

See also

Notes

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  2. ^ Huggett, Nick (2024), "Zeno's Paradoxes", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Spring 2024 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-03-25
  3. Diogenes Laërtius, Lives, 9.23 and 9.29.
  4. Parmenides 128d
  5. Parmenides 128a–b
  6. (, Diogenes Laërtius. IX Archived 2010-12-12 at the Wayback Machine 25ff and VIII 57).
  7. ^ Aristotle's Physics Archived 2011-01-06 at the Wayback Machine "Physics" by Aristotle translated by R. P. Hardie and R. K. Gaye
  8. "Greek text of "Physics" by Aristotle (refer to §4 at the top of the visible screen area)". Archived from the original on 2008-05-16.
  9. Benson, Donald C. (1999). The Moment of Proof : Mathematical Epiphanies. New York: Oxford University Press. p. 14. ISBN 978-0195117219.
  10. ^ Brown, Kevin. "Zeno and the Paradox of Motion". Reflections on Relativity. Archived from the original on 2012-12-05. Retrieved 2010-06-06.
  11. ^ Moorcroft, Francis. "Zeno's Paradox". Archived from the original on 2010-04-18.
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  15. Huggett, Nick (2010). "Zeno's Paradoxes: 3.1 The Dichotomy". Stanford Encyclopedia of Philosophy. Archived from the original on 2022-03-01. Retrieved 2011-03-07.
  16. Huggett, Nick (2010). "Zeno's Paradoxes: 3.2 Achilles and the Tortoise". Stanford Encyclopedia of Philosophy. Archived from the original on 2022-03-01. Retrieved 2011-03-07.
  17. Aristotle. "Physics". The Internet Classics Archive. Archived from the original on 2008-05-15. Retrieved 2012-08-21. Zeno's reasoning, however, is fallacious, when he says that if everything when it occupies an equal space is at rest, and if that which is in locomotion is always occupying such a space at any moment, the flying arrow is therefore motionless. This is false, for time is not composed of indivisible moments any more than any other magnitude is composed of indivisibles.
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  30. Aristotle. Physics. Vol. VI. Part 9 verse: 239b5. ISBN 0-585-09205-2. Archived from the original on 2008-05-15. Retrieved 2008-08-11.
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References

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