Thales is probably the oldest Western philosopher that we have record of, so in that sense he "invented philosophy" (although I'm sure there were people before him that asked the same kinds of questions, whose work was lost). His theory, basically, was that all that exists is composed of a single substance: water. Today we know that water is composed of other things, but the idea that everything is only different modalities of a single thing is still alive, whether that thing is a certain kind of boson, or quantum fields, or whatever. How Thales would respond to questions about whether abstract concepts like "infinity", or subject experiences like "sadness" were also simply made of water we don't know, but those kind of questions are probably what motivated Plato to have a more complex theory of existence beyond simply matter, in his theory of forms.

Anaximander and Anaximenes where students of Thales who worked on similar problems, who also believed in a kind of monist system, i.e. that reality was made of only a single thing.