Anaximander.

Like Thales, Anaximander was a monist. But he rejected Thales' supposition that water is the material archê. Instead, he proposed the apeiron (the indefinite, or the infinite). Why did he do this? There is only one extant fragment (6 = B1). It was recorded by the commentator Simplicius (6th C.), who was preserving an account of Anaximander given by Aristotle's student Theophrastus; it's possible that Simplicius may have gotten the quote from yet another commentator, Alexander, in …

…Guthrie, pp. 89-90. Assessment of Anaximander: 1. A response to a (perceived) logical difficulty in Thales' theory. 2. Postulation of a theoretical entity to explain observable phenomena. 3. The postulation of something beyond experience was not new (cf. the gods). What was new: what is postulated is not personified or anthropomorphic. It is a kind of matter. 4. Problem: how can the apeiron contain the opposites it gives rise to and still be a simple unity? Cf. Guthrie, p. 87.