Aristotle drew an analogy between the "foamy matter" (τὸ ἀφρῶδες) found in nature and the "seed" of an animal, which he viewed as being a kind of foam itself (composed, as it was, from a mixture of water and pneuma). For Aristotle, the generative materials of male and female animals (semen and menstrual blood) were essentially refinements, made by male and female bodies according to their respective proportions of heat, of ingested food, which was, in turn, a byproduct of the elements earth and water. Thus any creature, whether generated sexually from parents or spontaneously through the interaction of vital heat and elemental matter, was dependent on the proportions of pneuma and the various elements which Aristotle believed comprised all things. While Aristotle recognized that many living things emerged from putrefying matter, he pointed out that the putrefaction was not the source of life, but the byproduct of the action of the "sweet" element of water.