in his two books. But it does make me a bit queasy. It seems odd to suppose that I was once a millimeter long. The oddity is enough to make me take a very close look at the competing views. (There are also worries about defending abortion, but I'm going to make a concerted effort to set them aside. It's a good general principle that we should not let ethics steer metaphysics, but vice versa.)
The Constitution View, the view defended by Lynne Rudder Baker in Persons and Bodies, avoids the oddity by saying that I am a person merely constituted by an organism. The organism is one thing, and it did begin to exist as a fetus. But the person is another, and it began to exist only at the point when self-awareness emerged. I am a person, not an organism, and I never existed as a fetus. The problem is, I find aspects of that view completely unintelligible, as I explain here.
How else can we avoid saying I was once a millimeter long? I think maybe this is what I thought, before I really thought a lot about it-- I started to exist as a baby, or maybe as a late-term fetus. That would have been roughly in the month of March, since I was born in May. I didn't exist in February, January, December, etc., but of course "my" early term fetus did. So what did I used to think about the connection between my early term fetus (F1) and my late term fetus (F2)? I used to think F1 became F2, of course. Duh!
This, then, raises a question of logic: If F1 becomes F2, must there be an underlying entity that endures, first as F1 and then as F2? In other words, must it be the case that F1=F2? If that is the case, and I think I did exist as F2, then I'm forced to think I existed even earlier, as F1. In other words, my old way of thinking about this is untenable. I can't say I started to exist as a baby, and deny ever having been a fetus.
So... what is the logic of becoming? Is it really true that if A becomes B, then there's some underlying, enduring entity, so that in fact A=B? Well, it's at least often true. When a child becomes a teenager, the identity holds: the teenager is the child. When caterpillar Charlie becomes butterfly Bob, Bob is Charlie. Is there any such thing as a case of becoming, where A becomes B, so there's a very intimate connection between A and B, yet it's not true that A=B?
Don't say "a frog becoming a prince" if you mean the frog disappears and a prince takes his place. Because that's not the sort of "becoming" that's involved when an early term fetus becomes a late term fetus. We need a very intimate relation, where the properties of F1 are pretty continuous with the properties of F2, yet there's some coherent reason to acknowledge two separate entities. Is becoming ever that way--a transition from one entity to a bonafide second entity? If you can think of an example like that, pray tell.