Identity: tracking bodies

Introduction: The problem of personal identity is important as well as interesting / frustrating.

One approach to personal identity relies on the identity of their body.

But what does the identity of a body, as physical object, consist in?

One theory: an object retains its identity only insofar as the components that make it up retain their identity. This is the "mereological" theory. ("Mereology: the logic of the relationship of part to whole" - Blackburn's wonderful Dictionary*).

Objection: Heraclitus' river, as construed by Plato. It is absurd to think that 'you cannot step into the same river twice'.

But if identity can be retained even though components come and go, can a thing change all its components and still stay the same thing?

"Heraclitus was exceptionally haughty and supercilious and ... eventually became a misanthrope who lived in the mountains and fed on grasses and plants." Diogenes Laertius, quoted in Diane Collinson, 50 Major Philosophers, London, 1987, Routledge, p.10.

The story of the Ship of Theseus brings out the difficulty of thinking this.

Plutarch's version:

""The ship wherein Theseus and the youth of Athens returned 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."

The story can be developed to hammer difficulties with the mereological theory home.

The Steptoe argument:

I We have good reason to say that the ship in the museum is the same as the same as the ship Theseus sailed in.

II We have good reason to think that the Steptoe rebuild is the ship Theseus sailed in.

III But if A is the same as B and

A is the same as C then

B is the same as C

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If we accept I and II we have the conclusion that the ship in the museum is the Steptoe rebuild.

Another theory: Spatio-temporal continuity:

"A persisting object must trace a continuous path through space-time." Cohen, Identity, Persistence, and the Ship of Theseus, http://faculty.washington.edu/smcohen/320/theseus.html (Cohen offers here brilliantly concise and clear notes on the problem of identity.)

The Spatio-temporal continuity theory is of course not without difficulties of its own.

VP

*The Oxford Dictionary of Philosophy, Oxford, 1994, OUP.

VP