The present essay takes up matters discussed by Robert Pasnau in his response (published in the same issue of Philosophy) to our previous criticism of his account of Aquinas's view of when a foetus acquires a human soul. We are mainly concerned with metaphysical and biological issues and argue that the kind of organization required for ensoulment is that sufficient for the full development of a human being, and that this is present from conception. We contend that in his criticisms (...) of our account Pasnau fails clearly to distinguish first, between a passive potentiality and an active capacity; second, between having a power intrinsically and being an instrumental agent of that which has it intrinsically; third, between per accidens and per se causal series; and fourth, between sense cognition and conceptual thought. We conclude that philosophy and embryology support the position that human beings exist from the point of conception. (shrink)