And does this mean that the soul is infused at conception? If not, then can the conceptum really be called human except equivocally? Can it be said that a teaching that the soul is infused at conception can be found in the Deposit of Faith? It seems not--it appears that in the past, some reason has been given for the prohibition of abortion other than that the conceptum is a individual human being, and therefore abortion is murder and unjust. What, then, do recent papal pronouncements that abortion ends human life mean with regards to the status of the conceptum? To this Thomist, it appears that to hold that the conceptum is human and a rational soul is nonetheless absent (or that the soul may not be infused at conception) is incoherent.
Some contemporary authors such as Robert George have argued that it can be shown from science that the conceptum is human. It seems to me that at best these arguments do show material continuity but do not rule out the possibility that a [brute] animal soul is present and is responsible for development, rather than a rational soul. One could posit some principle of parsimony, and that a transient soul is not necessary, as the rational soul could be responsible for development and much more (and therefore a rational soul is present from the very beginning and hence infused at conception), but it seems very difficult (impossible?) to justify this assumption of parsimony. (Would Ockham's razor actually lead to the conclusion that a brute soul is present and not a rational one?)
If this reasoning concerning the identity of the conceptum is correct, then to claim that one can know from natural reason that the conceptum is a human life at conception is wrong, unless, again 'human' is being used equivocally. And, by extension, the natural law argument for the prohibition cannot definitively be founded upon the injustice of murder.
[It should be noted though that for the materialist, considerations about rational and irrational souls do not matter. As the rational soul is not needed as a formal cause defining something to be human, one could give a (false) argument that the conceptum is human, based on causality and continuity. But this seems to be a dishonest way to persuade someone of the evilness of abortion.]
