Some brief remarks are necessary. Or more accurately, a question: Where does the regress end?
If potential is morally significant, then we must, logically, consider the moment of human conception the fundamental dividing line. As a biological fact, zygotes and fetuses are members of the species Homo sapiens. If allowed to develop, a zygote has the potential to become an adult human being. There is no chance that this same zygote will develop into an adult cow, or an infant chicken. Consistency would require, then, that at the moment of conception (some) rights should be extended to this simple fusion of cells because of its potential to be a rational actor.
Celebration by those who condemn all abortion is unwarranted given the question begging nature of defining “moral persons” so irrationally. Surely our intuition tracks the other way: Does my interest in not being experimented upon while conscious only count if I’m a human? Think about what make’s vivisection “bad” (pain, for example), and then ask yourself: Are human animals the only species who can experience pain? As a concrete hypothetical, consider the existence of extraterrestrial life. If we were visited by alien life (assuming they aren’t hostile of course), to what criterion would we appeal to decide if they ought to count, morally? Given the principles we hold regarding our treatment of the mentally handicapped, for example, making the ability to reciprocate obligations the necessary characteristic doesn’t follow. And membership in our species would exclude them entirely.
For those who wish to avoid the perfectly logical conclusion of this argument from potential, an adjustment is necessary to somehow include all born human beings, including the non-rational and non-self-aware, and exclude all nonhuman animals in a manner that doesn’t parallel the justification for blatant sexism – i.e., that doesn’t track along the species barrier.
The common response is predictable: a slide of definition. Our critics begin with “potential” (i.e., group membership in the species Homo sapiens) because they know that they must include all human animals and then conclude with an entirely different criterion for “moral persons” (i.e., rationality), which excludes every human baby ever born. This shift is subtle, but not clever or logical.
“…the potential for autonomy no more justifies treating someone as if she has autonomy any more, and probably less, than does the fact that one’s potential for dying justifies treating her as if she were dead. Philosopher Joel Feinberg thinks allocating rights based on potential is simply a logical error.”
– Steven M. Wise