| There is no evidence scripturally for a preexistent human being apart from the body so my presumption is that when the sperm and ovum become a new independent human life the immaterial part of man is present as well. That's somewhat of a conjecture. But my sense of things is that the most reasonable way to understand the soul in relationship to the human being is that at the time a human being is created and comes into existence, all aspects of that human come into being. There is no reason to dichotomize the human life and the soul. It strikes me as the tidiest and most reasonable thing that when a being comes into being, that being is everything that it is.
Having a soul is not a property like having skin color or gender. Having a soul is something that defines what the substance of a human being is. Each human being is a certain type of being and that includes a physical and nonphysical element. When that new being comes into being it will be what it is in its entirety, even though aspects of its being develop later.
It is a mistake to talk about human beings as being potential persons. Personhood is part of the substance of being human, not a property a human develops later. Personhood isn't like hair color that you can change, it's not a property. It is part of the substance of what it is to be human. Trying to distinguish between the humanness and the personhood is looking at it in an inaccurate way. Personhood is part of the substance of being human, it doesn't develop at some other time. It strikes me as the most reasonable way to look at it is that when a being comes into being it is everything substantively that it is. There are properties that it requires. But everything else, its humanness, is there the moment it comes into being. If it comes into being as a separate living thing at the fertilization of the egg, which is what we understand biologically is the beginning of a new life, then at that point it is reasonable to conclude that it gets its soul. It gets everything then and develops from that stage.
The argument that an acorn isn't an oak but a potential oak isn't true. An acorn is an oak just as a mature oak tree is an oak, both are oaks but they are at different stages of development. It is entirely what an oak was meant to be. No nascent form of a being is the same as the mature form. Is an infant an adult? Of course not. Is a little sprig of an oak tree coming out of the ground a mature tree? No. An infant form is not an adult form, that's all you're saying when you say that an acorn isn't an oak. An acorn is an oak but it's not the adult form. In the same way, a brand new human being formed by the fertilization of a human egg and sperm is everything that a human being is in its substance or essence. That would include being soulish.
Fertilization is not an arbitrary point because that's when a new being comes into being by biological definition. This distinction between a property and a substance is very important because properties are what substances have. Properties can grow and change, but that's different than the essence itself. Any particular thing has all those things which define it as the particular thing because if it didn't have those things it wouldn't be that particular thing.
A square always has four sides. What if it had only three sides? It wouldn't be a square, it would be something else--a triangle. If a human didn't have a soul it wouldn't be a human being because that's what human beings are. You can't say that one being gains a property that changes it from one being to another. All beings have the same profile of what they are and they gain and lose and develop properties over time. But that doesn't change who or what they are. | 
