From the Metaphysics. Though Thales would have been familiar with ancient legends about water progenitors—including, as Hesiod reports in his Theogony, how Tethys “bore swirling rivers to her mate, Ocean”—Aristotle is clear in the Metaphysics that Thales’ views were not derived from mythology but constituted the foundation of natural philosophy. The first of the ancient Greek Seven Sages to have received that title, Thales originated the Milesian school of philosophy. He died in 546 BC, one year before his disciple Anaximander.
Of the first philosophers, most thought the principles that were of the nature of matter were the only principles of all things. Just so they say nothing else comes to be or ceases to be; for there must be some entity—either one or more than one—from which all other things come to be, it being conserved.
Yet they do not all agree as to the number and the nature of these principles. Thales, the founder of this type of philosophy, says the principle is water (for which reason he declared that the earth rests on water), getting the notion perhaps from seeing that the nutriment of all things is moist, and that heat itself is generated from the moist and kept alive by it (and that from which they come to be is a principle of all things). He got his notion from this fact, and from the fact that the seeds of all things have a moist nature, and that water is the origin of the nature of moist things.
Some think that even the ancients who lived long before the present generation, and first framed accounts of the gods, had a similar view of nature; for they made Ocean and Tethys the parents of creation, and described the oath of the gods as being by water, to which they give the name of Styx; for what is oldest is most honorable, and the most honorable thing is that by which one swears.