Heraclitus of Ephesus, an important city just to the north of Miletus, meditated on the ideas of his predecessors.  In his enigmatic pronouncements, he hinted that if one substance turns into another—for instance if air turns into fire and water, and so on, then nothing, or rather no substance, is permanent.  If A turn into B and B turns into C, and then, in turn, C turns back into B and B into A, what sense does it make to identify some type of substance, for instance B, as more real than the others?  Heraclitus uses fire rather than air or water as his basic reality.  But he seems to choose fire for its symbolic value rather than its permanence.  For fire is the most obviously unstable and impermanent of all stuffs.  It shows us how everything is always changing.  And yet, the structure of the world stays the same.  In fact, for all the impermanence of substance, Heraclitus holds that the world itself, the cosmos, is uncreated: it always exists.  How?  Because the changes in one direction (from A to B to C) are balanced by the changes in the opposite direction (from C to B to A).  As he puts it, the road up and the road down is one and the same.  There is a deep orderliness to the universe embodied not in the surface changes, but in the balance of things changed.[7] 

Heraclitus explained the heavenly bodies as being like bowls of fire, presumably with the fiery contents being spherical in shape.  As the bowl of the moon rotated monthly, one would see the form of a crescent, a half moon, a gibbous moon, a circular moon and, when the opaque bowl was turned toward the earth, the moon would disappear at the end of the lunar month.  An eclipse would be caused by a rapid rotation of the bowl of the sun or the moon.[8]  The sun was “new every day,” presumably because the fire in the bowl was extinguished every night and fed by new fuel (vapors from the earth) the next day.[9]

[7]. Heraclitus DK22B31, B36, B60, B76.  See Graham 1997.

[8]. Heraclitus A12.

[9]. Heraclitus B6.