St. Thomas explains in his commentary on the Metaphysics that of the seven sages of Greece, who followed the three theological poets (Orpheus, Museus, Linus), six focused their studies on morals, while Thales investigated the causes of things. Thales is, therefore, the father of speculative philosophy.
The names of the seven wise men are as follows. The first was Thales of Miletus, who lived during the time of Romulus and when Achaz, King of Israel, was reigning over the Hebrews. The second was Pittacus of Mitylene, who lived when Sedecias was reigning over the Hebrews and when Tarquinius Priscus was reigning over the Romans. The other five sages were Solon of Athens, Chilo of Lacedaemon, Periander of Corinth, Cleobulus of Lydia, and Bias of Prienne, all of whom lived during the period of the Babylonian captivity. Hence, since Thales alone among these men investigated the natures of things and distinguished himself by committing his arguments to writing, he is described here as the originator of this science.St. Thomas, On Metaphysics