A turning point in the succession of cosmologies occurred when Parmenides of Elea, a Greek colony in southern Italy, wrote a poem criticizing cosmology and arguing—explicitly presenting arguments for the first time—that change was impossible and all that existed was “what is.”[10] Scholars disagree over whether he allowed only one thing to exist, his “what-is” or whether he thought that there could be different things that “are.” In any case, Parmenides seemed to be disturbed by the philosophy of Heraclitus that claims that everything is always changing (he evidently missed the point that for Heraclitus the balanced changes preserve the structure of the world).[11] In response he argued that there could be no change in the real world. What-is is, and there what-is-not is not. Differentiation and change presuppose what-is-not, which is an incoherent notion, so at some basic level, reality is changeless.
Yet he offered his own version of cosmology in the second half of his poem, though he may have meant it as worthy only of a consolation-prize. In his cosmological account, there were two types or “forms” of reality, Light and Night, with contrasting properties. Light is rare, bright, and buoyant, while Night is dense, dark, and heavy. By mixing them together one could produce every kind of stuff we encounter.[12] Parmenides offered the first theory of elements, where those elements have fixed properties or essences. By combining with other elements (in this case the one other element in different proportions), they produced the multiplicity of resultant compounds we encounter in the world. Parmenides went on to develop a cosmological theory based on this “chemistry.” In his cosmology he claimed the moon was illuminated by the sun, and gave a reason: its shining face was always turned towards the sun.[13] That brilliant insight would later change everything for Anaxagoras, who used it to figure out the structure of the heavens.
If Parmenides was right, the moon’s perceived shape should vary with its position relative to the sun. At the beginning of the lunar month, the moon appears as a thin crescent in the west, just above the setting sun, its convex side facing the sun. The next night it is a little higher and farther east at sunset, and also appears as a slightly thicker crescent. Each night it grows larger as it moves left relative to the setting sun, until about seven days into the lunar month it appears as a half-moon in the south. Then on succeeding nights it becomes fuller as a gibbous moon until it appears as a completely circular full moon rising in the east as the sun sets in the west. The moon continues rising later at night and setting later in the morning, going through the opposite stages as it draws nearer the rising sun, now with the luminous part of the moon facing east toward the rising sun.
[10]. Parmenides DK 28B2, B6, B7, B8.
[11]. See Graham 2002.
[12]. Parmenides B8.50-61, B9, B12.
[13].Parmenides B14, B15.