Thales

"Thales has been identified as the first person to investigate the basic principles, the question of the originating 

''substances of matter and, therefore, as the founder of the school of natural philosophy. Thales was interested in almost everything, investigating almost all areas of knowledge, philosophy, history, science, mathematics, engineering, geography, and politics."'' - Patricia O’Grady

What we actually know about Thales is based on the testimony of a series of decidedly biased ancient sources, but the stories they've left us are an incredible window into the past, thousands of years ago. They also present the figure of a legendary scholar and innovator. The tendency to treat Thales as the first natural philosopher begins with Aristotle whereas Plato might have attributed such a title to Socrates or some other now forgotten figure... it's hard to say. Even if he's not the first he's certainly a viable early example of philosophical thinking and as O'Grady suggests, he is indeed attributed with commentary on a wide variety of interests.

Testimonia:
Practical activities

RW-Mi-T3; KRS-66; DK-11A6 "The story goes that Croesus did not know how his troops were going to cross the river, since the bridges I mentioned were not in existence at the time. But Thales was in the camp, and he helped Croesus by making the river flow on both sides of the army, instead of only to the left. This is how he did it, they say. He started upstream, above the army, and dug a deep channel which was curved in such a way that it would pass behind the army’s encampment; in this way he diverted the river from its original bed into the channel, and then, once he had got it past the army, he brought it back round to its original bed again. The immediate result of this division of the river was that it became fordable on both sides." Herodotus, Histories

RW-Mi-T7; KRS-72; DK-11A9 "The story about Thales is a good illustration, Theodorus [illustrating the detachment of the philosopher from the humdrum reality of the world]: how he was looking upwards in the course of his astronomical investigations, and fell into a pothole, and a Thracian serving-girl with a nice sense of humour teased him for being concerned with knowing about what was up in the sky and not noticing what was right in front of him at his feet." Plato, Theaetetus

Cosmology - typically understood as the study of the nature and activity of the universe, including the earth.

RW-Mi-T1; KRS-64; DK-11A5 "The Lydians and the Medes even once fought a kind of night battle. In the sixth year, when neither side had a clear advantage over the other in the war, an engagement took place and it so happened that in the middle of the battle day suddenly became night. Thales of Miletus had predicted this loss of daylight to the Ionians by establishing in advance that it would happen within the limits of the year in which it did in fact happen." - Herodotus, Histories

RW-Mi-T4; KRS-76; DK-11A17 "In his Astronomy Eudemus reports... that Thales was the first to discover the eclipse of the sun and the fact that the period of its solstices is not always equal." Eudemus in Theon of Smyrna, Mathematics useful for reading Plato

RW-Mi-T11; KRS-89,91; DK-11a22 "Thales too (as far as we can judge from people’s memoirs) apparently took the soul to be a principle of movement, if he said that the stone has soul because it moves iron... Some say that the universe is shot through with soul, which is perhaps why Thales too thought that all things were full of gods." Aristotle, On the Soul Cosmogony - any model concerned with the coming-into-existence or origin of the universe, including the earth.

RW-Mi-T8; KRS-85; DK-11A12 "Most of the original seekers after knowledge recognized only first principles of the material kind as the first principles of all things. For that out of which all existing things are formed––from which they originally come into existence and into which they are finally destroyed––whose substance persists while changing its qualities, this, they say, is the element and first principle of all things... However, they disagree about how many of such first principles there are, and about what they are like. Thales, who was the founder of this kind of philosophy, says that water is the first principle (which is why he declared that the earth was on water); he perhaps reached this conclusion from seeing that everything’s food is moist, and that moisture is the source and prerequisite for the life of warmth itself (and the source of anything is the first principle of that thing).

"So, as I say, it was perhaps this that led him to reach this conclusion, and also the fact that the seeds of all things have a moist nature (and water is the first principle of the moist nature of moist things). And there are people who think that those in the dim, distant past who first began to reason about the gods, long before our present generation, shared this conception of the underlying nature; for these poets made Ocean and Tethys the parents of creation, and claimed that the gods took their oath upon water––the river Styx, as the poets call it." Aristotle, Metaphysics

Cosmography - the branch of science which deals with the general features of the universe, including the earth.

RW-Mi-T5; KRS-78; DK-11A3A "Victory went to Thales, Whose cleverness showed not least in the fact that He is said to have measured the tiny stars of the Wain, By which the Phoenicians sail." Callimachus, Iambus

RW-Mi-T9; KRS-84; DK-11A14 "Others say that the earth rests on water. This is the oldest account that has been passed down to us today, and they say it was the view of Thales of Miletus, that the earth stays where it is as a result of floating like a piece of wood or something similar (for none of these things is so constituted as to keep its position on air, but they do so on water) -- as though the same argument did not apply to the water supporting the earth just as much as to the earth itself. After all, water is just as incapable of staying suspended in mid-air, and is also constituted as to keep its position only when it is on something." - Aristotle, On The Heavens

RW-Mi-T10; KRS-88; DK-11A15 "Thales says that the world is held up by water and rides on it like a ship, and that what we call an earthquake happens when the earth rocks because of the movement of the water." - Seneca, Questions about Nature

RESOURCES
Fragments and Testimonia

H. Diels, Die Fragmente der Vorsokratiker, 3 vols., ed. W. Kranz, 6th edn. (Zurich: Weidmann, 1951–2).

G. S. Kirk, J. E. Raven, and M. Schofield, The Presocratic Philosophers, 2nd edn. (Cambridge: Cambridge University Press, 1983).

R. Waterfield, The First Philosophers (Oxford: Oxford University Press, 2000).

D. W. Graham, The Texts of Early Greek Philosophy, 2 vols. (Cambridge: Cambridge University Press, 2010).

Thales Specific

R. M. Dancy, ‘Thales, Anaximander and Infinity’, Apeiron, 22 (1989), 149–90.

D. R. Dicks, ‘Thales’, Classical Quarterly, 9 (1959), 294–309.

J. Mansfeld, ‘Aristotle and Others on Thales, or the Beginnings of Natural Philosophy’, in [29], 126–46 (first pub. Mnemosyne, 38 (1985) ).

A. Mosshammer, ‘Thales’ Eclipse’, Transactions of the American Philological Association, 111 (1981), 145–55.

D. Panchenko, ‘Thales’ Prediction of a Solar Eclipse’, Journal for the History of Astronomy, 25 (1994), 275–88.

D. Roller, ‘Thales of Miletus: Philosopher or Businessman?’, Liverpool Classical Monthly, 3 (1978), 249–53.

Online

http://www.iep.utm.edu/thales/

http://www.philosophy.gr/presocratics/thales.htm