Thales was the founder of the physicalist stream of philosophy which attempted to explain the world in materialist terms. The foundation of this philosophy is described as arkhe or the basic principle of all things. This principle has matter being the original source from which all things come and in which all things are made. Matter, being composed of elements, is always preserved and, for Thales, the basic component of all matter is wáter. Aristotle says that Thales drew his conclusión that wáter is arkhe from his observation of nature. According to Aristotle, Thales argument would have been that all food contains moisture, therefore what nurtures and sustains all things must substantially be wáter.
The difficulty with this theory was that Thales had to justify how all things could come into being from water and ultimately return again to the originating material. For Thales, the matter of the universe consisted of physiological, biological, meterological and geological states. Metallurgy had been practised well before Thales’ hypotheses, therefore Thales knew that all things could be reduced to a liquid state. Water also readily changed into the three states of matter: gas, liquid, solid. His understanding that water could transform into earth was able to be observed in his town of Miletus, which was situated on the Meander Delta. This historically observable phenomena showed that river mouth had silted so much that wharves had to be rebuilt in the port town. This coming-into-being of land would have substantiated Thales view that everything was derived from water. So, when we consider that our exploration of other planetary bodies depends upon their ability to have water, the assertion that water is the principle of all things still resonates.