Monday, July 13, 2009

The Ship of Theseus

"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, Theseus

Plutarch thus questions whether the ship would remain the same if it were 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? Consider also, "George Washington's axe" (sometimes "my grandfather's axe") which is the subject of an apocryphal story of unknown origin in which the famous artifact is "still George Washington's axe" despite having had both its head and handle replaced.

One solution to this paradox may come from the concept of four dimensionalism. Ted Sider 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. An object is made up of a series of causally related time-slices. And the whole aggregate of time-slices, namely the 4-dimensional object, is as single object. But the individual time-slices can have qualities that differ from each other.

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