Thales was born in the city of Miletus in Ionia around 685 B.C. He was a well- known public figure in his day and was included on most lists naming the Seven Sages of Greece. It seems that his fame was not only due to his theoretical achievements, but also to more practical triumphs. Among his accomplishments he could count military engineering (he redirected the flow of a raging river so that King Croessus's army could cross), geometry (he devised a means of measuring the height of the pyramids and the distance of ships at sea using triangulation), and astronomy (in 585 B.C., using his astronomical knowledge along with the Babylonian tables of lunar and solar orbits, he was the first man to correctly predict a solar eclipse). He also wrote a guide for mariners and managed to amass a fortune by using his astronomical theories to predict the appearance of a large olive crop and then buying up all the olive presses in the surrounding area (the sole aim of this latter exploit, supposedly, was to prove that philosophers can make money if they want to). In between these various activities Thales also found the time to develop the first known philosophical system. Unfortunately, no fragments of his original writings have survived to the present day, and all that we know about his thought boils down to five statements found in Aristotle. From these five statements we can identify four basic tenets of Thales' world view: (1) The world derives from water; (2) The world rests on water; (3) The world is full of gods; (4) Soul produces motion. Aristotle offers up even these snippets very hesitantly, suggesting that even by his time Thales was known only by report and not through any first-hand evidence.
With this notion, Thales ushers in the single most important preoccupation of the Presocratics: the problem of physis. In its most robust form, the physis problem is a search to identify that thing out of which all else is derived and will ultimately return (the source or origin of the world), as well as that thing of which everything else is a variation (the unifier within nature). In Thales', as far as we can tell from the evidence, his physis—water—only fulfills the first of these functions. Water is the substance from which the entire cosmos emerged (and perhaps also to which it will return). Whether or not it is also the unifier within nature is impossible to determine, since we have no evidence that bears on the issue. Everything in the world may be a variation on water according to Thales, or it may not.
The claim that there is a single substance out of which everything else derived is commonly referred to as material monism: material because it claims that the source of all nature is something physical (as opposed to, say, something mental), and monism because it posits that there is only one such thing. Thales is the first of a long line of material monists, extending all the way to the present day. The notion that the whole universe emerged from primeval water was a common theory in Near Eastern and Greek mythology, so Thales' idea is not original in this sense. What is original about the idea, though, is that Thales' claim is about water as a natural phenomenon and not about water as some personified god or goddess. In addition, Thales seems to have provided rational arguments for his water-centric view. According to Aristotle, Thales turned to biology in order to understand how the universe was produced. In the biological world he observed three things: first, he observed that all life depends on water. Give water to a plant and the plant will survive, remove the water and it will whither and die. This was, similarly, the case for all animals. Further he observed that seeds, the source of most life, are themselves moist. Finally, he observed that even heat (in the form of the sun and moon) is generated out of moisture and kept alive by it (apparently this last bit of data was based on the relationship he observed between heavenly bodies and the oceans). Observing that life springs from water in the biological sphere, he concluded that the same must hold true of the entire universe.
Hippolytus gives another possible line of reasoning that might have convinced Thales. Water, alone among the natural elements, can take form of a solid, a liquid, or a gas. He claims that Thales noticed that, "As the water solidifies, things acquire firmness, as it melts their individual existence in threatened."
What seems most likely is that Thales' first formed the notion of a water- derived world from the mythological water cosmogonies and that he then turned to these confirming instances in biology and proto-chemistry for support.
The notion that the earth floats on water was a commonly held mythological belief, and it is not clear that Thales' himself held it. Some commentators have argued that the attribution of this belief to Thales' is the result of confusion about his real claim, which was that the world derives from water. The question of the world's support, though, is one that the other Milesian philosophers address, and so it seems likely that Thales addressed it as well and that this was his answer.