Of Thales we know virtually nothing. However, Aristotle’s comment on his thought in-and-of itself is worthwhile:
For there must be some nature, either one or multiple, out of which the other things come into being while that one is preserved. About the number and kind of such sources, however, they do not all say the same thing, but Thales, the founder of this sort of philosophy, says it is water (for which reason too he declared that the earth is on water), getting hold of this opinion perhaps from seeing that the nourishment of all things is fluid, and that heat itself comes about from it and lives by means of it (and that out of which things come into being is the source of them all). So he got hold of this opinion by this means, and because the seeds of all things have a fluid nature, while water is in turn the source of the nature of fluid things (~ Metaphysics 983b17 – 28).
There’s more here than “Thales said everything is water.” “Water” is like a principle that is a substance. It preserves (“nourishment of all things”), causes motion (“heat,” [is where] “things come into being”), and is a point of origin (“seeds… have a fluid nature”). Thales and “everything is water” is a way of examining how a cosmology is comprehensive. It has to account for how time “works:” how things change in it, how they are preserved in it, where they come from.
There are some who think that very ancient thinkers, long before the present age, who gave the first accounts of the gods, had an opinion of this sort about nature. For they made Ocean and Tethys the parents of what comes into being, and made the oath of the gods be by water, called Styx by them; for what is oldest is most honored, and that by which one swears is the most honored thing. But whether this opinion about nature is something archaic and ancient might perhaps be unclear, but Thales at least is said to have spoken in this way about the first cause. (One would not consider Hippo worthy to place among these, on account of his cut-rate thinking.) (~ 983b28 – 984a5)
Did Thales actually do science (natural philosophy)? In the Politics, Thales is used as an example of a philosopher who isn’t a a philosopher. Asked what the worth of philosophy is, he used astronomical calculations to determine if there might be a good harvest and invested accordingly. That’s not even an attempt to find the truly good for himself, let alone the comprehensive good the Politics inquires about.
Here, Thales is saying something that seems secular in proclaiming “all is water.” It is a break, in a way, with the poets and the mythical tradition. Water is not necessarily “Ocean and Tethys.” Then again, Ocean and Tethys are water and tied very closely to the gods for a number of Greeks. The gods themselves are swearing on water. Perhaps “water” is an elaborate comment on how the gods (beings? originators of beings?) relate to chaos (water). But it is more likely the case that myth allowed for this sort of “scientific” supposition easily. The opinion that “everything is water” was in a way implicit in myth and the one who could articulate it first and sermonize about it would be honored greatly. “Hippo” is a comment on what happens when philosophers compete for honor. If you want the truth, you have to move away from honor and riches and to an account that actually explains something. “Everything is water” is near impossible to distinguish from “everything is chaos” in both a mythical and scientific setting.
Aristotle, Metaphysics. trans. Joe Sachs. Sante Fe: Green Lion Press, 2002