Starting off my course in Ancient Greek Philosophy I am very much enjoying Beginning With the Pre-Socratics by Merrill Ring, a brisk little book that moves quickly into serious philosophical issues and sticks there, very nice discussion of Parmenides for example. (I've also drug out my old buddy The Presocratic Philosophers by Kirk and Raven, always a treat, and On Reserve for the students are Irwin's Classical Philosophy and Kenny's Ancient Philosophy, both thematically arranged instead of the usual chronological treatment of the ancients.)
Anyway, I noticed from Merrill Ring's book a nice connection between the Milesian Anaximander and Heraclitus that I probably should have noticed long ago but hey. Runs like this: the basic problem is to explain why change occurs, or why action in general occurs. To say, "Because there is energy coursing through the world" is to express a type of theory, it turns out to be a good one so far as it goes, but the Ionians were approaching the question of energy at a more basic level. "What generates and organizes this energy?" Anaximander's idea was that there were sets of opposite qualities (dry/wet, cold/hot) that generated change as they struggled with each other. The tension between the opposites is the source of the energy. If these qualities were essentially linked with physical elements - (dry:earth/wet:water), (cold:air/hot:fire) - then maybe we could generate a systematic explanation of change. Our one existing fragment of Anaximander reads, "Existing things perish into those things out of which they have come to be, as must be; for they pay reparation to each other for their injustice according to the ordenance of truth."
Or as Heraclitus says, "Things taken together are whole and are not whole, something which is being brought together and brought apart, which is in tune and out of tune; out of all things there comes a unity, and out of a unity all things." Specifically one of Heraclitus's doctrines is the unity of opposites: "And as the same thing there exists in us living and dead and the waking and the sleeping and young and old;: for these things having changed round are those, and those having changed round are these" (trans. from Kirk and Raven). So the treatment of "opposites" in Anaximander and Heraclitus is the same: they are the right concepts to use to understand the energy that is causing change. Furthermore, both Ionians are trying to practice "physis," that is to discover basic principles of material interactions. Noticing the connection to Anaximander makes Heraclitus look more materialistic: the unity of opposites is an attempt to develop a dynamic model of nature rather than a static one.