According to Aristotle, De gen. animal., and therefore according to St Thomas, no bodily matter of the male semen ever becomes a constituent of the body of the embryo: that is entirely taken from the mother. What the male semen furnishes is a certain motive power, dunamis kai kinêsis, which causes conception and carries the embryo through the stages of its development. This is the 'formative power' here spoken of. Offspring is said to be of father and mother, "as a couch is of a carpenter and timber" (De gen. animal., I, xxi), the male semen being as the tool, which, wielded by the carpenter, makes the couch, but is not the material of which the couch is made. So (I, xxii): "The male semen is no part of the embryo: . . . . but nature uses it as an instrument and actually efficient cause, as is the efficiency of tools in products of art." Pursuant to this doctrine, Aristotle expresses himself in a way not unfavourable to traducianism in regard of the sentient soul: -- "The body is from the female, but the soul is from the male, for the soul gives formal being to a certain body"; esti de to men sôma ek tou thêleos, hê de psuchê ektou arrenos; hê gar psuchê ousia sômatos tinos esti (II, iv): which is explained (II, v), "the female supplies the material, but the male the principle of motion": hulên men oun parechei to thêlu, tên de archên tês kinêseôs ho arrên. In the same De gen. animal., II, v, Aristotle goes on to say that the soul which the male parent imparts is not the vegetative soul, -- for that is already in the material supplied by the female, -- but the sentient soul: empoiei gar touto (to arren) tên aisthêtikên psuchên hê di hautou ê dia tês gonês. A sentient soul, he adds, is necessary from the first, for the formation of what is to be not a mere vegetative but a sentient body.
Of God and His Creatures: 2.88