The soul is the form of the body. It contains all the information that forms a unique person—you. This means it must be present at conception, because at conception a new living human person comes into being and begins to develop.
Precisely. The soul is not some ephemeral floaty thing that’s sort of attached to the body, as many Protestant denominations teach and most of American culture simply
assumes. This theory of the soul/mind/body problem is called “dualism.”
Catholics instead subscribe to a theory called “hylomorphism,” which posits that the soul and the body are unique, but are
radically unified, such that one is incomplete without the other. They depend on one another in a very special way. Thus, to talk of the soul forming “before” or “after” the body forms actually makes no sense – it’s like talking about “square circles” or “hot ice” or “military intelligence.”
This is a very refined philosophical point, though, and to really grasp it would require several years of serious study. The theory originates with Aristotle, who put a unique spin on Plato’s theory of forms, and it comes to its full blossom with the work of Thomas Aquinas. A general overview can be gotten from Ralph McInerny’s
A Brief Glance at St. Thomas Aquinas, which covers both philosophers, but real understanding would require lots of reading of both philosophers, along with other modern books to help guide you along the way.
But the simple answer to your question is the one everyone is giving: you are ensouled at the moment of fertilzation, when two non-human organisms (sperm and egg cells) meet, interact, and become a single, fully human organism.
