Aquinas famously thinks that there is a succession of forms in utero, with first a vegetable form, then an animal form, and then a rational animal (human) form. No two of these forms are had simultaneously.
But if a three-dimensionally extended object that has form A comes to be a three-dimensionally extended object that has form B, with no other forms intervening, then, in every inertial reference frame except for at most one, there is a time at which some of the matter has form A and some of the matter has form B. So unless there is a privileged frame, Aquinas’s story doesn’t work.
In the following diagram, the slanted dashed line indicates a reference frame where some of the matter has form A (red) and some has B (blue).
Here is a variant that could work, but does not seem very plausible. We could imagine that when we have a transition from A to B, the matter of A, except at one point, passes to B through one or more other forms, indicated by the yellow portion of the diagram. These might be forms of mere particles, or they could be some special forms. In other words, A dies off into a point, with the dead matter acquiring transitional forms, and then B starts growing from the last point of A, incorporating the transitional forms. Where the A and B substances meet will be a point either of A or of B, but not of both. The narrowing of A and the growth of B happen at the speed or light or less.
On this variant, no inertial frame contains both A and B points. (If light moves at 45 degrees from horizontal in the diagram, then inertial frames correspond to lines like the dashed one making a less than 45 degree angle with the horizontal.)
But there is something rather weird going on here. Suppose that A is the vegetable form and B is the animal form (a similar argument will apply if A is the animal form and B the human form). Then close to the pointy meeting between A and B, the yellow stuff contains the vast majority of what biology would call “the embryo”, and a fairly well-developed one, since it’s on the cusp of becoming an animal. Yet the vast majority of that “embryo” is the yellow stuff—neither the vegetable nor the animal, but something else, maybe mere atoms. Indeed, once we get close enough to the meeting point, the yellow will materially function just like an embryo, since a tiny subatomic hole makes no difference to material functioning. This is very odd, and gives us reason to reject Aquinas’s story.
Of course, the main alternative to Aquinas’s story is that the gametes change into a human being. That faces some of the same difficulties. However, I think the difficulties are less if the gametes are not themselves a substance, but a plurality of substances, perhaps particle-substances. In the diagram below, the gamete-stuff is in yellow, and the blue indicates the human being. We still have the problem that early on most of what we have will need to be biologically very close to a functioning zygote, and yet it is in yellow, except for a small blue hole corresponding to where ensoulment is spreading out from a single point. But I think this is less problematic, because at this juncture the yellow stuff is something that is less obviously an organism. (Admittedly, the blue stuff is less obviously an organism when it is nearly a point. But what makes it an organism is that while its matter has little going for it, it’s got the right form.)
So, relativity makes it hard to hold on to Aquinas’s embryology. Which is a nice thing for pro-life Thomists who want to defend ensoulment at conception and hence deny Aquinas's embryology--or Catholic Thomists who find the doctrine of the Immaculate Conception incompatible with that embryology.
Maybe there is some way of getting out of this by using the considerations from yesterday’s post, though.
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