Consider that there is a logically possible world in which some non-rational animal evolve in such a way that it has all the physical features necessary to be classified as homo-sapien, and yet it would not have a rational soul. Alternatively it is logically possible that some species evolves which looks nothing like human beings do and which has a rational soul and bears the image of God (imagine big black cats). Given that these are both logically possible, how exactly do we tell the difference between that which has a rational soul and that which is animate but does not have a rational faculty? The knee-jerk reaction to this question is often to say that it is obvious which creatures have a rational faculty and which do not – the use of true syntactic language, the concept of the ‘self’ as an abstract subject related to predicates, the possibility of feeling existential angst in response to the world – these all act as markers of the rational faculty, as they are the peculiar results of rational reflection. Putting aside the controversial issue of ape-language studies (I maintain that no impressive evidence has ever been produced which should convince us that whatever ‘language’ apes have it is analogous to true syntactic language), let’s focus on the opposite problem of ‘dumb’ (lit.) human beings. Suppose somebody is mentally handicapped, and cannot exhibit any signs of having a rational faculty – on what grounds do we treat such a thing as a human person with a rational soul? One cannot make any ad hoc argument from biological genus, since, as the thought experiments above indicated, it is possible for some human-body to belong to a non-rational animal, and there is no necessary connection whatever between the human-body and the rational soul.
I think we rightly presume that such a being is a human person the ‘soul’ of whom is in the genus ‘rational’ precisely because of the origin of that person is known to be from within the human community (particularly the sexual communion of a man and a woman). Strictly speaking, our belief that every human person conceived in the womb has a rational soul is based on analogical reasoning and/or religious/philosophical dogma. If we are Catholic, for instance, then we know by faith that at the moment of conception God immediately creates the rational soul – it may be impaired by unfortunate physical disposition, and thus the rational faculty may be impossible for that person to exercise in any way to which we would have empirical access, but nevertheless the human being would be a ‘person’ precisely because of their substance belonging to the rational genus. Philosophical reasons for believing this may come from a kind of rationalist philosophy of sex which would entail that that which two members of a species conceive of must be of the same order and nature as the parents. Ultimately the view that every offspring of human intercourse is itself a human person with a ‘rational’ soul has to be founded on an anthropology which views community as the quintessential foundation, in some sense, of human persons. This criteria is satisfied in the Catholic philosophy of sex, a portion of which is expounded in blessed John Paul II’s theology of the body, which itself will say that the procreative quality of the marital act is itself always an iconic participation in the mystery of the divine life of the Trinity in which the Holy Spirit proceeds from the Father in communion with the Son. Like proceeds from like, and so likewise with human procreation does like proceed from like.
The fascinating question then becomes, I think: what of engineered and animate human bodies which are in no way proximately the result of two parents, and which also bear no clear signs of having a rational faculty – do we assume that these are human persons, or do we not? I think we should not.
Interestingly this may be a good reason for Catholics to distance themselves from Leibniz’ monadology, precisely because according to Leibniz a monad belongs in the rational genus because of the predicates which attend to it, but those are themselves the results of aggregate-relations in such a way that it would be conceivable for Leibniz that a human person be engineered merely by the proper arrangement of matter.