There is something about practical things that knocks us off our philosophical high horses. Perhaps Heraclitus really thought he couldn't step in the same river twice. Perhaps he even received tenure for that contribution to philosophy. But suppose some other ancient had claimed to have as much right as Heraclitus did to an ox Heraclitus had bought, on the grounds that since the animal had changed, it wasn't the same one he had bought and so was up for grabs. Heraclitus would have quickly come up with some ersatz, watered-down version of identity of practical value for dealing with property rights, oxen, lyres, vineyards, and the like. And then he might have wondered if that watered-down vulgar sense of identity might be a considerably more valuable concept than a pure and philosophical sort of identity that nothing has.

John Perry, introduction to Identity, Personal Identity, and the Self