As I have previously discussed here, a substance is a particular being, something that exists in and through itself. It must be a being that is whole in itself and distinct from all other particular beings. Let us then ask ourselves the following question: Is the being that comes into existence at conception a particular being?
Yes. It is a distinct being that is whole in itself, not a part of another being. If this were false, then any and every part of the being that comes into existence at conception would be a part of the pregnant woman. Thus, pregnant woman would eventually be two headed, four armed, four legged &c. Some of them would even be hermaphrodites. This is absurd. Thus, the being that comes into existence at conception is a particular being.
If this is so, the question that arises next is: What type of being is this particular being? It is human. It is a being that posses a full human genome. Science can tell us that.
The next question we must ask is: Is this newly existing human being a person? A person is an individual substance of a intellectual nature. We have already demonstrated that this being must be taken as a substance rather than as an accident or as something that is solely a part of another being. And since we are talking about a particular being rather than the nature of a particular type of being taken as a genus, we must admit that it is an individual substance.
Now we are left to ask whether of not this being has a intellectual nature. This being has been shown to be human. A human being by definition possesses human nature.
So what is human nature? How do we come to know the essence of humanity? Well, how do we come to know the essence of anything? We observe its acts and properties. These show us its powers, which in turn lead us to understand its essence.
So what powers to humans have. There are the basic powers of bodily life: the powers of nutrition and growth which preserve the individual, and the power of generation which preserved the species. There are the sensitive powers of sight, touch &c, which gives a being perception of individual things. And there is the power of intellect, which gives a being perception not just of individuals but also of universal natures. Now, if it is part of human nature to possess an intellect, it follows that human nature is an intellectual nature.
Human beings are persons, since every particular human being is an individual substance of an intellectual nature. The being that comes into existence at conception is a being that possesses a human nature. It follows necessarily then that the being that comes into existence at conception is a person.