Identity

How do we know our right hand from our left hand when there is literally nothing that can be said about one which cannot be said about the other? -ace

Comments

That is a great question! Of course, all of the relations among the parts of my right hand are the same as the relations among the parts of my left hand. (We might have to imagine slightly idealized hands here, since someone might happen to have a cut on their right hand but not on their left.) But there are relations to external things that differ between your right and left hands. For instance, my left hand is currently nearer to the "a" key on my typewriter than my right hand is. Because my hands can stand in different relations to other things, I can learn which is "right" and which is "left". For example, my mother presumably held my right hand and not my left when she said "That's your right hand, Marc." In this way, I learned which of my hands was "right".

However, let's turn from the question of how we know which hand is right and which is left (an epistemological question -- a question about knowledge) and ask a metaphysical question (a question about reality). Suppose there were a universe that was utterly empty throughout its history except for a hand (unattached to any body) floating in it. (Pretty gruesome, but let's not think too hard about that!) Would that hand be a right hand or a left hand? Now we cannot appeal to the hand's relations to other things to give it its handedness, since there are no other things. Would it be neither a right hand nor a left hand? Both? What would make it one rather than the other?

Two important historical discussions of this problem. First, Leibniz in the Third Letter to Clarke, par.5. Interestingly, Leibniz asks whether the entire cosmos could be reversed (its handedness changed). Second, Kant, Prolegomena, par. 13, where he calls handed object 'incongruent counterparts'. In both cases, the question is on what grounds, if any, the distinction could be made.

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