From Chapter I., The Milesian School

16. The Primary Substance is Infinite
Anaximander's reason for conceiving the primary substance as boundless was, no doubt, as indicated by Aristotle, "that becoming might not fail."⁶⁷ It is not clear, however, that these words are his own, though the doxographers speak as if they were. It is enough for us that Theophrastos, who had seen his book, attributed the thought to him. And certainly his view of the world would bring home to him the need of a boundless stock of matter. The "opposites" are, we have seen, at war with one another, and their strife is marked by "unjust" encroachments on either side. The warm commits "injustice" in summer, the cold in winter, and this would lead in the long run to the destruction of everything but the Boundless itself, if there were not an inexhaustible supply of it from which opposites might continually be separated out afresh. We must picture, then, an endless mass, which is not any one of the opposites we know, stretching out without limit on every side of the world we live in.⁶⁸ This mass is a body, out of which our world once emerged, and into which it will one day be absorbed again.

67. Phys. Γ, 8. 208 a 8 (R. P. 16 a). Cf. Aet. i. 3, 3 (R. P. 16 a). The same argument is given in Phys. Γ, 4. 203 b 18, a passage where Anaximander has just been named, τῷ οὕτως ἂν μόνον μὴ ὑπολείπειν γένεσιν καὶ φθοράν, εἰ ἄπειρον εἴη ὅθεν ἀφαιρεῖται τὸ γιγνόμενον. I cannot, however, believe that the arguments at the beginning of this chapter (203 b 7; R. P. 17) are Anaximander's. They bear the stamp of the Eleatic dialectic, and are, in fact, those of Melissos.

68. I have assumed that the word ἄπειρον means spatially infinite, not qualitatively indeterminate, as maintained by Teichmüller and Tannery. The decisive reasons for holding that the sense of the word is "boundless in extent" are as follows: (1) Theophrastos said the primary substance of Anaximander was ἄπειρον and contained all the worlds, and the word περιέχειν everywhere means "to encompass," not, as has been suggested, "to contain potentially." (2) Aristotle says (Phys. Γ, 4. 203 b 23) διὰ γὰρ τὸ ἐν τῇ νοήσει μὴ ὑπολείπειν καὶ ὁ ἀριθμὸς δοκεῖ ἄπειρος εἶναι καὶ τὰ μαθηματικὰ μεγέθη καὶ τὰ ἔξω τοῦ οὐρανοῦ· ἀπείρου δ' ὄντος τοῦ ἔξω, καὶ σῶμα ἄπειρον εἶναι δοκεῖ καὶ κόσμοι. The mention of σῶμα shows that this does not refer to the Atomists. (3) Anaximander's theory of the ἄπειρον was adopted by Anaximenes, and he identified it with Air, which is not qualitatively indeterminate.