The Lagoon: How Aristotle Invented Science

by Armand Marie Leroi

WHEN THE MENSES contact semen they congeal into an embryo or an egg. Aristotle uses homespun analogies to explain how this works: ‘The case resembles that of fig-juice which curdles milk, for this too changes without becoming any part of the curdling masses.’ Or, elsewhere, ‘this acts in the same way as rennet acts upon milk’. This is all about making cheese. When rennet, a substance derived from the stomachs of unweaned calves, is mixed with milk it causes them to separate into solid and liquid parts: curds and whey. Aristotle supposes that seminal pneuma does the same thing to menstrual fluid, coagulating the earthy stuff out of it, leaving a fluid behind. He probably thought the analogy particularly apt. The active ingredients (semen, rennet, fig-juice) all draw their power from being charged with vital heat; their substrates (menstrual fluid, milk) are both very closely related derivatives of blood.* The result of all this cheese-making is an embryo enveloped by a membrane floating in a fluid. And now the real business of pneuma begins: the manufacture of the embryo’s parts. Aristotle claims, and repeats the claim with the vigour of a man who thinks he’s made a major discovery, that the heart is the first organ to appear in the embryo. It’s a reasonable one if we allow that he means the first visible functioning organ and so exclude the somites and notochord that form well before the heart does. It isn’t just a fact for Aristotle: it fits with theory too. The heart mus...