Human soul exists in various degrees. In the fetus it starts at the level of vegetal soul having properties of nutrition & growth. Then it becomes animal soul with features of movement and perception. With intellectual maturity it becomes a rational soul. The human body-soul unit can be envisioned as a continuous spectrum on the one end of it is the physical body followed by vegetal soul, then animal soul and finally rational soul. The various faculties of human soul do not make soul a composite structure instead these faculties are actualized in the soul such that there is unity of soul’s attributes and its essence
Actually, I had an SSPX-affiliated priest explain this to me, and to others in a study group, in much the same way — first a vegetal soul, then an animal soul, then a rational soul. Only when the rational soul is infused, does the fetus become a human person, the termination of whose life would be murder.
However, and this is a BIG “however”, we might hold this opinion, which is rooted in Aristotle and later Aquinas, but that is all it is — just an opinion, just a philosophical argument. What if it’s wrong? What if Almighty God does, in fact, infuse an immortal soul from the moment of conception, in spite of any fine arguments otherwise? As far as being a matter of moral certitude, we do not know, nor can we know. We know that an immortal soul is not in either the sperm or the egg. We know that the immortal soul exists at the point when a fetus could survive outside the womb, and for some time before that. As far as drawing a line, the only thing that makes sense is to assume that the immortal soul exists at conception — just as a hunter assumes that a noise in the distance could be a person and not a deer unless he or she knows otherwise.