The Lagoon: How Aristotle Invented Science

by Armand Marie Leroi

Aristotle once again invokes that mysterious stuff, pneuma. It’s not only an instrument of the sensitive soul, but also a component of the inheritance system. Aristotle searches semen for the signs of activity. He finds it in the fact that semen resembles foam – or does so immediately after ejaculation. The foam is due to a charge of pneuma introduced by concoction of the semen during sex. Pneuma does not, however, need to be carried in semen since in those insects with strange sex it gets injected directly into the female. The upshot is a theory for how an animal’s soul is reproduced in the embryo. The structure of the father’s soul is, in effect, encoded in his semen by pneuma-tic action.* We must not think of pneuma as the carrier of genetic information itself: it’s not Aristotelian DNA. Rather, Aristotle’s units of inheritance are much more abstract; they’re the movements that pneuma induces in the semen. When he describes the motive principle in semen, the movement that is the future soul incarnate, he chose a word both apt and elegant: aphros or foam; and he meant by this both literally the foam visible in semen, and the foam visible in the wash of waves receding from a shore. Yet, as the passage makes clear, in choosing this word he was also thinking of something else. It is also for aphros, he says, that the Goddess of Love was named.