Thales is the father of ancient Greek philosophy insofar as he was the first that raised the point that a material substance explains all the natural phenomena. He was born about 624 BCE in Miletus and he considered the founder of the Ionian School, also called the Milesian school. Thales was an avid traveler as Hieronymus of Rhodes indicates in his report that Thales measured the pyramids by their shadow, having observed the time when our shadow is equal to our height. For the ancient Greek Sages of the sixth-century (for example Solon, see Timaeus) it was a custom to visit Egypt and studding the traditional fountain-head. Proclus, in Euclidem, mentions that "Thales left Egypt and went to Greece to further his study of geometry"(1). Thales was regarded as one of the "Seven Sages" of ancient Greece. He died at an old age when watching athletic matches due to heat exhaustion. The inscription on his tomb is: Here in a narrow tomb great Thales lies; Yet his renown for wisdom reached the skies.(1)

Thales was the first Greek philosopher to speculate about the primary material element of all beings and cosmic phenomena, which he identified as water. The importance of water in life and nature was probably the principal reason that made Thales came to this conclusion. In Orphic mythology and cosmogony we find Water and Earth as one of the first cosmic elements of the Cosmos creation. Damascius in "de principiis" notes that "The Orphic Theology which is said to be according to Hieronymus and Hellanicus (if indeed he is not the same man) is as follows: water existed from the beginning, and is the matter from which earth was solidified."(1) Water, Air, Fire or any other principle was for the Presocratics the root of life, soul and generally the power of the living nature. This power the ancient Greeks called Fiesthe.

Thales was not only a great philosopher but also a great astronomer and mathematician. Heraclitus says that "Thales was the first come to astronomical conclusions". After a long series of empirical observations Thales foretold the eclipse of the sun in 585 BCE and he wrote epic rhymes for the cosmic spheres. Diogenis Laertius in his 1st book "Lives and
Opinions of Eminent Philosophers" notes that "And some, including Choerilus the poet, declare that he was the first to maintain the immortality of the soul. He was the first to determine the sun's course from solstice to solstice, and according to some the first to declare the size of the sun to be one seven hundred and twentieth part of the solar circle, and the size of the moon to be the same fraction of the lunar circle... He is said to have discovered the seasons of the year and divided it into 365 days". (5) Plato in Theaetetus (174 ) describes an interesting story about Thales:
"Theodorus, a witty and attractive Thracian servant girl is said to have mocked Thales for falling into a well while he was observing the stars and gazing upwards; declaring that he was eager to know the things in the sky, but that what was behind him and just by his feet escaped his notice." As a mathematician, Thales, is famous for his theorems, three of which are attributed to him by Proclus -- circle bisected by diameter; angles at base of isosceles triangle are equal vertically opposed angles are equal. Herodotus provides important evidence for Thales' activities as statesman and engineer; he mentioned that "When he became to the Halys river, Croesus then, as I say, moved his army across by the existing bridges; but according to common account of the Greeks, Thales the Milesian transferred the army for him"(1)

Diogenis Laertius notes that "...according to some he left no book behind; for the 'Nautical Star-guide' ascribed to him is said to be by Phokos the Samian. Callimachus knew him as the discoverer of the Little Bear, and wrote as follows in his Iambs...; while according to some he wrote only two works, On the Solstice and On the Equinox, considering the rest to be incomprehensible"(1). Some fragments from Thales philosophy we can find in his students and in the next philosophers such as Aristotle (On the Soul).

Diogenis Laertius in his 1st book "Lives and Opinions of Eminent Philosophers" (4) & (5)
(1) Of all things that are, the most ancient is God, for he is uncreated.

(2) The most beautiful is Cosmos, because it is God's action.

(3) The larger is space, because it holds all things.

(4) The swiftest is mind, because it speeds everywhere

(5) The strongest is the necessity, for it matters all

(6) The wisest is the time, because he brings everything to light.

(7) The things you offer to your parents you must wait to get from your chills

(8) There is no difference between life and death

(9) Know thyself

(10) Someone asked him which is older, day or night and he replied "Night is the older by one day"

(11) Someone asked him who is the happiest man and he replied "The one who has healthy body, resourceful mind and a docile nature

Aristotle Metaf. A3,983b6 (1)
" ... for there must be some natural substance, either one or more than one, from which the other things come-into-being, while it is preserved. Over the number, however, and the form of this kind of principle they do not all agree; but Thales, the founder of this type of philosophy, says that it is water..."

Aristotle On Soul (1)
"Others says that the earth rests on water. For this is the most ancient account we have received, which they say was given by Thales the Milesian, that it stays in place through floating like a log or some other such thing (for none of these rests by nature on air, but on water) - as though the same argument did not apply to the water supporting the earth as to the earth itself"

Heraclitus Homericus Quaest. Hom. 22 (1)
"For moist natural substance, since it is easily formed into each different thing, is accustomed to undergo very various changes: that part of it which is exhaled is made into air, and the finest part is kindled from air into aither, while when water is compacted and changes into slime it becomes earth. Therefore Thales declared the water, of the four elements, was the most active, as it were, as a cause."

Seneca Qu. nat. III, 14 (1)
« For he [Thales] said that the world is held up by water and rides like a ship, and when it is said to 'quake' it is actually rocking because of the water's movement"

Aristotle de an. A5, 4II a7 (1)
And some say that it [soul] is intermingled in the universe, for which reason, perhaps, Thales also thought that all things all full of gods.

Diogenis Laertius I,24 (DK ii Ai) (1)
According to Thales the soul is immortal

Aetius I,7, II (1)
Thales said that the mind of the world is god, and that the sum of things is besouled, and full of daimons; right through the elemental moisture there penetrates a divine power that moves it.

Callimachus Iambus I,52, fr. 191 (1)
... for the victory belonged to Thales
who was clever in judgment,
not least because he was said to have measured out
the little stars of the Wain,
by which the Phoenicians sail.

Text

1. G.S.Kirk, J.E.Raven,M.Schofield., 'The Presocratic Philosophers', A Critical History with a selection of texts, Second Edition, Cambridge University Press [1995]

2. James Fieser., Text file adapted from passages in John's Burnet's <Early Greek Philosophy> [1892]

5. Diogenes Laertius, 'Lives of Eminent Philosophers', Volume 1, Translated by R.D. Hicks
LOEB CLASSICAL LIBRARY, Harvard University Press [1972]

Sources

3. The Cambridge Dictionary of Philosophy., Cambridge University Press [1995]

Translation

4. Giannis Stamatellos., The Presocratic Philosophers.

Attension to translation

6. James Fieser., Internet Encyclopedia of Philosophy