Per asked:

Dear experts,

I have been wondering for quite some time now. If I was conceived say a day or just an hour later than I actually was, what would become of ‘me’?

Answer by Craig Skinner

Wonder no more.

There would be no you. There would be somebody else, with your name (if of your gender), and he/she might be wondering what would become of him/her if conceived a day or an hour earlier.

The essential feature which individuates each of us as a particular human being is being the product of a particular sperm and ovum, thereby conferring metaphysical uniqueness, and genetic too (or co-uniqueness for identical twins). A being produced from a different sperm or egg would be somebody else.

So, YOU could not have been conceived at a different time or by a different parent.

This illustrates the huge improbability of your (or my) existence. Had a different sperm out of the millions competing to penetrate the ovum been successful on that fateful occasion, had your father been away on business on the day you were conceived, you would not exist. Also the huge fluke that your mother and father chose each other from all the alternative mates available to each. And it’s mind boggling to think that not a single one of your millions of forbears, going back over 3 billion years, failed to reproduce. If just one of your myriad fishy ancestors had been eaten by a bigger fish when young, you would not exist. But, like the national lottery, given that the jackpot has to be won, somebody wins it with millions to one odds against, so, given that you exist, somebody has to be you.

The uniqueness of each of us is stressed by Derek Parfit in his analysis of the effect of new policies on future generations. We often hear how future people will be adversely affected by our actions. But it is very doubtful that anybody will be affected. Changed policies alter behaviour, often in subtle ways, people marry later, or have children later, move around the country more, etc etc. And the upshot is that after 2 or 3 generations, all the people being born would not have existed under the old policies, whilst all those who would have been born had the change not been introduced, don’t get born. So nobody can be adversely affected, no matter how bad the future world is. Future people benefit by existing when they otherwise wouldn’t. And those who would have been born under a no-change policy suffer no adverse effect since nonexistent people can’t be affected in any way.

And the metaphysical uniqueness view is recognized in most accounts of possible worlds. Thus each of us is world-bound (to the actual world) and couldn’t exist in any other possible world. So that if I say that I might have been a good philosopher, putting it as ‘there is a possible world in which I am a good philosopher’, the person who is the good philosopher in that world is not me but my COUNTERPART.