FROM its beginnings Greek Astronomy, like Geometry, sought to model itself after the type of a rational science; having to explain physical facts, it tried to do so by physical causes, that is to say causes of the same nature as these facts.
To primitive peoples, celestial phenomena are divine, that is, they depend entirely on the more or less capricious will of divinities. Doubtless, as we have seen, the Egyptians and Chaldeans already possessed some amount of astronomical knowledge, but this knowledge consisted, after all, in ascertaining the periodicity of celestial phenomena, without giving any explanation of these.
From the first, Greek astronomy launched out in another direction, as the works of the Ionian school show. These works appear incredibly daring if we compare them with the religious beliefs of the Chaldeans and Egyptians.
Thales, for example, lays down as a principle that water is the unique element from which all things arise by the action of purely physical causes, for water can be solidified into ice, be changed into vapour, that is, air, etc. Having once laid down this principle, Thales deduces from it a cosmology which, in spite of its childish simplicity, remains physically rational.
However it was only with difficulty that Greek astronomy succeeded in specifying its ideal and object. It passed through a series of stages which may be