Arche is Aperion 
Of those who say that [the first principle] is one and moving and indefinite, Anaximander, son of Praxiades, a Milesian who became successor and pupil to Thales, said that the indefinite (to apeiron) is both principle (archē) and element (stoicheion) of the things that are, and he was the first to introduce this name of the principle. He says that it is neither water nor any other of the so-called elements, but some other indefinite (apeiron) nature, from which come to be all the heavens and the worlds in them; and those things, from which there is coming-to-be for the things that are, are also those into which is their passing-away, in accordance with what must be. For they give penalty (dikê) and recompense to one another for their injustice (adikia) in accordance with the ordering of time—speaking of them in rather poetical terms. It is clear that having seen the change of the four elements into each other, he did not think it fit to make someone of these underlying subjects, but something else, apart from these. (Simplicius, Commentary on Aristotle’s Physics 24, lines 13ff. = 12A9 and B1)Anaximander
Pseudo-Plutarch (12A10) says that “Something productive of hot and cold was separated off from the eternal at the genesis of this world and from this a sphere of flame grew around the air around the earth like the bark around a tree.” 

The reaction of hot and cold
Rather, relying on a semi-biological model, Anaximander claims that the apeiron somehow generates the opposites hot and cold (Stanford Online)