A soul is the form of a living being. A form is that which causes a particular being to be the type of being it is. A particular being is a being that is whole in itself and distinct from all other particular beings.
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.
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.
If what exists after conception is a particular human being, then it has a human form. And since human beings are living things, this is the same as saying that it has a human soul.
The "being" in "human being" means being per se, i.e. being that is whole and distinct in itself. A thumb does not posses this type of being. It is not whole and distinct in itself. Rather, it is a part that belongs to another being, a being that is whole and distinct in itself.
Form and matter are the two concomitant principles of physical beings. Form is that which makes a being be this type of being. Matter is what makes a being be this particular being. If you agree that there are individual beings, but that individual being share common types or natures, then you have all you need to hold the existence of form and matter. And since a soul is simply the form of a living being, you have all you need to hold the existence of soul.
There is nothing in this definition that requires any theological belief. There is nothing that requires a person to hold that any soul is spiritual and will survive the death of the form/matter composite being whose existence it informs. Indeed, Aristotle is not always clear on where he stands in this regard. Many believe that the textural evidence shows that Aristotle himself did not believe in the immortality of the human soul.
Nothing here requires faith. All that is required is the ability to recognize that many individual physical beings share the same type or nature. Biology does this whenever it divides living beings into Kingdom, Phylum, Class, Order, Family, Genus and Species.
taxonomy: Biology. the science dealing with the description, identification, naming, and classification of organisms.