My logic is as follows. We are tasked with creating a reasonable definition of personhood. To evaluate a candidate definition, it should exclude every obviously non-human thing, and include everyone obviously human. We cannot achieve perfection but we can place our decision on a more rational basis than simply saying, “such-and-such is a human begin because I say so;” a technique entirely void of rationality.
Let us consider some obviously non-human things: A rock, A carrot, A dog, My severed finger, A sperm in a man’s testicle and the ovum in his wife’s ovary.
A precondition for humanness is biological animal life. Clearly a rock or a carrot does not qualify in this regard. However a dog does qualify, so this qualification, while necessary, is not sufficient; it includes too many obviously non-human items.
Let us consider some obviously human beings: An ordinary adult, a baby, a mentally retarded child, a pair of identical twins.
Now, let us consider qualifications that include each of the obviously human items but (and this is the critical consideration) does not include any of the obviously non-human items.
Human genetic material includes all the humans, but includes my severed finger and the sperm and ovum. Unique genetic information includes the finger (from random mutations in the various cells). Unique and sufficiently distinct genetic information incorrectly excludes one of the pair of twins. Clearly it seems difficult to formulate a consistent rational definition based on genetics alone.
Sentience, the exhibition of obviously human mental processes, seems a more fruitful area of inquiry. But sentience alone will not serve; a baby does not display distinctively human mental processes. However, it posseses in present time the necessary physical apparatus of the brain to develop such consciousness, as well as the ability to in present time to exist independently.
If we attempt to include the indirect potential, we lose the ability to exclude things like the seperate sperm an ovum, which does indeed have the indirect potential.
I have summarized the argument and necessarily omitted many important points. For instance in some cultures an infant under one year old does not qualify as “obviously” human; infanticide is not considered murder. But the basic thrust of the matter in creating a definition is that one must craft it based on substantial agreement (obviousness), and then see where the situation in dispute lies. My analysis may be flawed, but I believe I have conclusively proven that it rational and not arbitrary.
The distinction between “arbitrary” and “rational” is not a matter of opinion or perspective. There are many rational decisions I disagree with in detail, but I oppose on principle all arbitrary decisions (when someone else is coercively applying that decision on another). I consider even more objectionable decisions and distinctions that are irrational and self-contradictory.
The rational argument against imposing arbitrary decisions and distinctions is that, simply put, there is no consistent means to separate valueable arbitrary decisions from obviously horrid ones.
One may simply say that rational, logical considerations are simply not sufficient for deciding which decisions to coercively enforce upon others. Obviously, such a position cannot be rationally debated; to such beings I can only say we have no basis for communication. We may coexist peacefully or engage in hostility, but we cannot communicate.