Concerning embryos, Porphyry takes an original view on issues that had been left undecided by his teacher Plotinus and earlier by the doctor Galen. What role is played in the development of the embryo by the souls or the natures of the father, of the mother, of the embryo, or of the whole world? Porphyry's detailed answer, in contrast to Aristotle's, gives a significant role to the soul and to the nature of the mother, without, however, abandoning Aristotle's view that the mother supplies no seed. In the fragments of On What is in Our Power, Porphyry discusses Plato's idea that we choose each of our incarnations, and so are responsible for what happens in our lives. This volume contains an English translation of the two commentaries, as well as extensive notes, an introduction and a bibliography.