Ship of Theseus
From Wikipedia, the free encyclopedia
The Ship of Theseus is a paradox also known as Theseus's paradox. It raises the question of whether an object, which has had all its component parts replaced, remains fundamentally the same.
Contents |
[edit] Variations of the paradox
[edit] Greek legend
According to Greek legend as reported by Plutarch,
| “ | The ship wherein Theseus and the youth of Athens returned [from Crete] had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their place, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same. | ” |
Plutarch thus questions whether the ship would remain the same if it was entirely replaced, piece by piece. As a corollary, one can question what happens if the replaced parts were used to build a second ship. Which, if either, is the original Ship of Theseus?
[edit] Heraclitus's river
The Greek philosopher Heraclitus is notable for his unusual view of identity. Arius Didymus quoted[1] him as saying:
| “ | Upon those who step into the same rivers, different and again different waters flow. | ” |
Plutarch also informs us of Heraclitus' claim about stepping twice into the same river, citing that it cannot be done because "it scatters and again comes together, and approaches and recedes"[2].
[edit] Locke's socks
John Locke proposed a scenario regarding a favorite sock that develops a hole. He pondered whether the sock would still be the same after a patch was applied to the hole. If yes, then, would it still be the same sock after a second patch was applied? Indeed, would it still be the same sock many years later, even after all of the material of the original sock has been replaced with patches?
[edit] Grandfather's old axe
"Grandfather's old axe" is a colloquial expression of unknown origin describing something of which little original remains: "it's had three new heads and four new handles but it's still the same old axe." The phrase has also been used in banter as in: "This is George Washington's original axe...", while holding up a patently new axe.
[edit] Other examples
One can think of many examples of objects which might fall prey to Theseus's paradox: buildings and automobiles for example can undergo complete replacement whilst still maintaining some aspect of their identity. Businesses, colleges and universities often change addresses and residences, thus completely "replacing" their old material structure for a new one, yet keeping the same purpose and often the same people that keep the organization functioning as it was. If two businesses merge, their identities merge (or one is consumed by the other). Similarly, the human body constantly creates, from materials consumed, new component parts, cells, as old cells die. Average age of cells in an adult body may be less than 10 years. [3]
If we relate identity to actions and phenomena, identity becomes even harder to grasp. Depending upon one's chosen perspective of what identifies or continues a hurricane, if a hurricane Evan collapses at a particular location and then one forms again at or near the same location, a person may be totally consistent to either choose to call the latter mentioned hurricane the same as the former (as in "Evan" was reinvigorated), or choose to call the latter a new hurricane "Frank" or "Georgia".
[edit] Proposed resolutions
[edit] Aristotle's causes
According to the philosophical system of Aristotle and his followers, there are four causes or reasons that describe a thing; these causes can be analyzed to get to a solution to the paradox. The Formal Cause or form is the design of a thing, while the Material Cause is the matter that the thing is made of. The "what-it-is" of a thing, according to Aristotle, is its formal cause; so the Ship of Theseus is the same ship, because the formal cause, or design, does not change, even though the matter used to construct it may vary with time. In the same manner, for Heraclitus's paradox, a river has the same formal cause, although the material cause (the particular water in it) changes with time, and likewise for the person who steps in the river.
Another of Aristotle's causes is the end or Final Cause, which is the intended purpose of a thing. The Ship of Theseus would have the same end, that is, transporting Theseus, even though its material cause would change with time. The Efficient Cause is how and by whom a thing is made, for example, how artisans fabricate and assemble something; in the case of the Ship of Theseus, the workers who built the ship in the first place could have used the same tools and techniques to replace the planks in the ship.
[edit] Definitions of "the same"
One common argument found in the philosophical literature is that in the case of Heraclitus's river we are tripped up by two different definitions of "the same". In one sense things can be qualitatively the same, by having the same properties. In another sense they might be numerically the same by being "one". As an example, consider two bowling balls that look identical. They would be qualitatively, but not numerically, the same. If one of the balls was then painted a different colour, it would be numerically, but not qualitatively, the same as its previous self.
By this argument, Heraclitus's river is qualitatively, but not numerically, different by the time one attempts to make the second step into it. For Theseus's ship, the same is true.
The main problem with this proposed solution to problems of identity is that if we construe our defintion of properties broadly enough, qualitative identity collapses into numerical identity. For example, if one of the qualities of a bowling ball is its spatial or temporal location, then no two bowling balls that exist in different places or points in time could ever be numerically identical. Likewise, in the case of a river, since it has different properties at every point in time—such as variance in the peaks and troughs of the waves in particular spatial locations, changes in the amount of water in the river caused by evaporation—it can never be qualitatively identical at different points in time. Since nothing can be qualitatively different without also being numerically different, the river must be numerically different at different points in time.
[edit] Four dimensionalism
One solution to this paradox may come from the concept of Four-dimensionalism. David Lewis and others have proposed that these problems can be solved by considering all things as 4-dimensional objects. An object is a spatially extended three-dimensional thing that also extends across the 4th dimension of time. This 4-dimensional object is made up of 3-dimensional time-slices. These are spatially extended things that exist only at individual points in time. An object is made up of a series of causally related time-slices. All time-slices are numerically identical to themselves. And the whole aggregate of time-slices, namely the 4-dimensional object, is also numerically identical with itself. But the individual time-slices can have qualitities that differ from each other.
The problem with the river is solved by saying that at each point in time, the river has different properties. Thus the various 3-dimensional time-slices of the river have different properties from each other. But the entire aggregate of river time-slices, namely the whole river as it exists across time, is identical with itself. Thus the 4-dimensional river is the same river as itself. So you can never step into the same river time-slice twice, but you can step into the same river twice.[4]
An apparent difficulty with this is that special relativity tells us that there is not a unique "correct" way to make these slices - any slice will do provided that all pairs of points in it lie along "time like" intervals. It is not meaningful to speak of a "point in time" extended in space. Special relativity does, however, ensure that "you can never step into the same river time-slice twice" no matter how you slice it. To do so one would have to step faster than the speed of light
[edit] In popular culture
The Ship of Theseus paradox is addressed in Terry Pratchett's Discworld novel "The Fifth Elephant". Here it is about an axe which periodically gets a new handle or a new blade. The characters in this book reason that, while it might not be the same axe physically, it will always remain the same axe emotionally. The Discworld series also pays homage to Heraclitus' statement by claiming that the (notoriously polluted and slow-moving) River Ankh in the city of Ankh-Morpork is the only river that it is possible to cross twice.
It is also referred to in the BBC comedy Only Fools and Horses. The road-sweeper character Trigger wins an award for using the same broom for many years and saving money. Upon being questioned further it is revealed however that the handle and head of the broom have been replaced several times, thus no material originally found in the broom was still on it. As Trigger is generally a dim-witted character and his remarks are laughed at, the comedy makes out that the broom is NOT the same one, or rather that no money was in fact saved.
In the 1986 book Foundation and Earth by Isaac Asimov, the ancient robot R. Daneel Olivaw says that over the thousands of years of his existence, every part of him has been replaced several times, including his brain, which he has carefully redesigned six times, replacing it each time with a newly constructed brain having the positronic pathways containing his current memories and skills, along with free space for him to learn more and continue operating for longer.
In The Restaurant at the End of the Universe by Douglas Adams, Marvin the android makes the same claim—with the exception of the diodes down his left side.
The Heraclitus's river paradox is featured in Francis Ford Coppola's Apocalypse Now Redux in the newly added french plantation scene in a dialogue between Captain Willard and french colonist Roxanne:
Roxanne: "Do you know why you can never step into the same river twice?"
Willard: "Yeah, 'cause it's always moving."
[edit] See also
[edit] References
- ^ Fr 39.2, Dox. gr. 471.4
- ^ "On the E at Delphi" 392b
- ^ Your Body Is Younger Than You Think
- ^ David Lewis,"Survival and Identity" (in Amelie O. Rorty [ed.] The Identities of Persons (1976; U. of California P.) Reprinted in his Philosophical Papers I.
