A key element of the cosmologies of the Milesian physiologi is the phenomenon of change. Each Milesian believed the entire universe was once composed of a single kind of matter. This primordial substance was called arche. For Thales it was water; for Anaximander it was an underlying substance he called Te Apeiron. For Anaximander's student Anaximines it was air. This arche then underwent change, becoming the diverse forms of matter we are familiar with today.
Change fascinated a later thinker name Heraclitus. Heracltius noticed things that were in a constant state of change. One of his many examples of this was a certain kind of potion. It consisted of barley mixed with wine. The barley, if left alone, always settles to the bottom. Thus separated out, the potion is no more, since its existence depends on a mixing of the two ingredients. Therefore, for the potion to persist, it must be stirred constantly. The potion is in the motion, as it were. The existence of the potion depends on ever-occurring change (stirring).
Heraclitus's most famous saying was, "You cannot step into the same river twice." This is because waters are constantly both leaving (from the mouth) and entering (from the sources) the river. So the second time you place your foot into a river, you are placing it into a different mass of water than you did before.
Heraclitus believed that the whole universe was like his potion and like rivers: in a constant state of flux. Presumably Heraclitus would not have been surprised at the modern understanding that the human body is constantly sloughing off cells and generating new ones, such that one's body is comprised of almost completely different individual cells from one decade to the next. If the whole universe is in flux what do we really mean when we refer to, for example, the Colorado River or Kevin Bacon? If we think we are referring to distinct material objects, then both the Colorado River and Kevin Bacon cease to be the moment after designating them as such. The Colorado River instantly becomes an assemblage of different water molecules from before, and Kevin Bacon instantly becomes an assemblage of different cells than before.
If all things are constantly in flux, what can it mean to "be" at all? Can we really know what anything "is" if all things are constantly changing?
Such questions, of the very meaning of being, are what comprise the philosophical field of ontology; and Heraclitus seems to have been the earliest known philosopher to have asked them. This began a long tradition whose latest inglorious highlight was President Bill Clinton musing over what the "definition of 'is' is." These questions would prove revolutionary in western philosophy. The Milesians and the Pythagoreans seemed to promise knowledge regarding the most fundamental workings of the universe itself. Heraclitus's conception of a world in constant flux threatened to take away not only that knowledge, but knowledge of even the most everyday facts of life. In this way, Heraclitus was something of a proto-skeptic. One of Heraclitus's followers, according to a rather defamatory story written by Plato, was so averse to saying anything certain, that he was reduced to saying nothing, and merely moving his finger back and forth.
Next in this series: The Ontological Counterrevolution: Parmenides, the First Extreme Rationalist