Anaximander.

…

… 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.