Probably the most interesting conversation I had at the UA's JFA exhibit was with an ex-nurse of an abortionist. [...] She thought it is a human's intellect and ability to understand that differentiates a human from an animal. She talked about her stay with Buddhists and how they told her that a mocking bird could repeat their chants but would not understand the words. Humans, however, would understand. The ex-nurse could not tell, however, when a human embryo first begins to understand and hence when the embryo is a human. I asked if the ability to understand is genetic, and I think she agreed. Then I commented that if it is genetic, that would make a fertilized egg a human being. She denied that one can know that for sure. She seemed to view viability outside the womb as the determining factor for humanness.As St. Thomas Aquinas says in his Summa Contra Gentiles, lib. 2 cap. 87 tit., "the human soul is brought into being through the creative action of God," not through the biological processes of procreation, which are instrumental causes, but as its efficient cause. In ibid., lib. 2 cap. 88 n. 3, Aquinas presents the argument of those who think the intellect arises from something genetic and not from an intellective or rational soul:
Moreover, as Aristotle [erroneously] teaches in the De generatione animalium [II, 3], the fetus is an animal before becoming a man. But, during the time in which the fetus is an animal and not a man, it has a sensitive [i.e., pertaining to the senses] and not an intellective soul; and, just as in other animals, this sensitive soul in indubitably produced by the active power of the semen. And yet that same sensitive soul is potentially intellective, just as that animal is potentially a rational animal; and the notion that the supervening intellective soul is substantially distinct from the sensitive one has been refuted already. It therefore seems that the substance of the intellective soul is derived from a power in the semen. [Viz., it is genetic.]He countered this argument in Summa Contra Gentiles, lib. 2 cap. 86 n. 4-5 and ibid., lib. 2 cap. 89 n. 3, saying:
[...] the intellective soul is the most perfect of souls and its power the highest [and] its proper perfectible subject is a body having many different organs through which its multifarious operations can be carried out; and that is why the soul cannot possibly be actually present in the semen separated from the body [...] The intellect, which is the proper and principal power of the intellective soul, is not the act of any part of the body, and therefore it cannot be divided accidentally as a result of the body's being divided [as through cell division]. Nor, then, can the intellective soul be so divided.Since the human soul includes the intellect, it follows that "the intellect is intrinsically independent from an organ" (XVII. of the 24 Thomistic Theses); therefore, it is not genetic. We know this, for example, because people with only one brain hemisphere still have a human intellect. But is there no relation between the brain and intellect? No, because there is a relation between body and soul even though the human soul does not depend on the body ("a human soul is incorruptible and immortal" and "subsists through itself," XV. of the 24 Thomistic Theses), just like form does not depend upon a specific matter; if it did and if the human soul is a human's form, a man, e.g., with a non-human body—i.e., with a different matter—would no longer be a man, a contradiction of our assumption that we are talking about a man.
Hence, from the hypothesis that the human soul is brought into being through the active power in the semen it follows that its being depends upon matter, as with other material forms. But the contrary of this has already been proved. The intellective soul, therefore, is in no way produced through the transmission of the semen.
And the hypothesis of the soul's presence in the semen from the beginning would entail the further consequence that animal generation takes place solely by way of partition, as with annulose animals, where two are produced from one. For, if the semen were possessed of a soul at the moment of its separation, it would then already be endowed with a substantial form. But in every case substantial generation precedes the substantial form; it never comes after it; and if any changes follow in the wake of the substantial form, they concern not the being but the well-being of the thing generated. Thus, the engendering of the animal would be completed with the mere alienation of the semen; and all subsequent changes would have no bearing upon the process of generation.
But this theory would be even more ridiculous if applied to the rational soul. For, first, the soul cannot possibly be divided as the body is, so as to be present in the separated semen; and second, it would follow that in all extra-copulative emissions of semen, without conception taking place, rational souls would nevertheless be multiplied.
Since the "intellect, which is the proper and principal power of the intellective soul, is not the act of any part of the body;" the brain is not the cause of thoughts, which are objects of the intellect; the brain, being a sense organ, is only instrumental to them. The soul is the cause of thoughts. But what if the intellect is the act of every and/or all parts of body? Could theories of holism and non-separability in physics disprove this, or are they based on false philosophies? No, they are most likely false philosophies because an intellective soul's "proper perfectible subject is a body having many different organs through which its multifarious operations can be carried out."
Also, from the definition of soul as an "immaterial part," the body cannot equal the soul. But why is there a soul in the first place? Asking this question is like asking: Why should I believe in anything non-physical pertaining to me? Thoughts, e.g., are non-physical; and undoubtedly they do pertain to you. We cannot escape the reality of non-physical entities such as thoughts, souls, angels, etc.; viz., there is a supernatural order.
Then what makes a human zygote a human, or possibly more specifically, a human scientist or physicist? That God created his soul and infused it into his body at conception.