No writings by Thales were extant even in the time of Aristotle, and it is believed that he wrote nothing. His philosophy, if we can call it by that name, consisted, so far as we know, of two propositions. Firstly, that the principle of all things is water, that all comes from water, and to water all returns. And secondly, that the earth is a flat disc which floats upon water. The first, which is the chief proposition, means that water is the one primal kind of existence and that everything else in the universe is merely a modification of water. Two questions will naturally occur to us. Why did Thales choose water as the first principle? And by what process does water, in his opinion, come to be changed into other things; how was the universe formed out of water? We cannot answer either of these questions with certainty. Aristotle says that Thales "probably derived his opinion from observing that the nutriment of all things is moist, and that even actual heat is generated therefrom, and that animal life is sustained by water, ... and from the fact that the seeds of all things possess {22} a moist nature, and that water is a first principle of all things that are humid." This is very likely the true explanation. But it will be noted that even Aristotle uses the word "probably," and so gives his statement merely as a conjecture. How, in the opinion of Thales, the universe arose out of water, is even more uncertain. Most likely he never asked himself the question, and gave no explanation. At any rate nothing is known on the point.