The earliest surviving record of a clash between the two cosmologies is in Plato's Phaedo – perhaps an unexpected place to find it, since the Phaedo is not about the natural world, but about the human soul, its destination after death, and the implications of immortality for the life of man or earth. And yet Plato's discussion of the issue gains an important dimension from its unexpected context, as we shall see, although he himself does not explicitly draw attention to the point.

Socrates is sitting in an Athenian prison waiting for the death sentence to be carried out. He talks with friends and explains the ground for his confidence that a man's soul survives his death. They listen and are convinced – but two of them express lingering doubts. The first doubt is quickly disposed of, but the second, says Socrates, is more troublesome. The first part of his attempt to allay it contains the critique of other philosophers that interests us. We shall return later to the doubt itself and the role of this critique in putting it to rest.

In his youth, Socrates says, he was an avid student of the philosophy of nature. ‘It seemed to me a superlative thing – to know the explanation of everything, why it comes to be, why it perishes, why it is’ (96a).

The kind of question he considered then, he says, was whether the growth of animals came from a fermentation of the hot and the cold, and whether the blood, or air, or fire,