Full Idea
If a boat can continue to exist after the planks that currently make it up have ceased to exist, and if the planks could continue to exist when the boat does not, then a boat cannot be identified with the planks that make it up at a given time.
Gist of Idea
If you can have the boat without its current planks, and the planks with no boat, the planks aren't the boat
Source
John Heil (Philosophy of Mind [1998], Ch.2)
Book Reference
Heil,John: 'Philosophy of Mind' [Routledge 1998], p.41
A Reaction
This seems obvious, but it opposes Locke's claim that the particles of an object are its identity. Does this mean identities are entirely in our heads, and not a feature of nature? I want to resist that.