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 his now lost commentary on Aristotle's Physics. Here is the fragment:
They pay penalty and retribution to each other for their injustice in accordance with the ordering of time.
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