For it is no more fitting for what is established at the center and equally related to the extremes to move up rather than down or sideways. And it is impossible for it to make a move simultaneously in opposite directions. Therefore it is at rest of necessity. Testimonia A26.
This passage, reported to us by Aristotle in On Heavens, provides Anaxagoras's explanation of what holds the earth steady. This account marks the first known use of the famous principle of sufficient reason, which asserts that there is no effect without a cause, i.e. nothing changes unless there is a reason for it to change. Because, on Anaximander's view, the cosmos is perfectly symmetrical, and the earth is placed squarely in its center, there could not possibly be any reason for the earth to move in one direction rather than another. Therefore, the earth does not move. Anaximander was the only early philosopher, as far as we know, to correctly claim that the earth is not supported by anything material, such as water (Thales), a cushion of air (Anaximines), or a large column (Heraclitus).