I thought it fitting to start by talking about Thales of Miletus, one of the most significant pre-Socratic thinkers in early philosophical history, he was famed for his theories concerning metaphysics.
Very little is known of the life of Thales- we know that he was born and lived in Miletus, which is now Turkey, and anecdotal evidence suggests that he was highly involved in business and politics. He had great importance as a key philosopher in history, Aristotle and Diogenes Laertius later wrote in great detail about him, Aristotle even proclaimed him to be the first philosopher in Greek tradition. Even more recent philosophers, such as Bertrand Russell recognize that "Western philosophy begins with Thales.
Thales was the first teacher of the Milesian School of philosophers; his pupil, Anaximander, who taught Anaximenes, who then, in turn, taught Pythagoras, later expanded his theories.
Unlike other philosophers at this time in history, Thales sought to explain natural phenomena, not through idiosyncratic theories of supernatural gods, but instead through rational explanation of the natural world around us - for example, he theorized that earthquakes are caused, because the earth is floating in water and earthquakes are waves in the water.
Thales is best known for his cosmological thesis, that fundamentally was, in the words of Aristotle in Metaphysics, "That from which is everything that exists and from which it first becomes and into which it is rendered at last, its substance remaining under it, but transforming in qualities, that they say is the element and principle of things that are." Thales believed that this substance must be something out of which everything else could be formed, essential to life, capable of motion and capable of change – which he concluded to be water.
Of course modern science can certainly disprove this theory, however this is still prominent in the evolution of metaphysics; not long after, Anaximenes (585 BCE- 528 BCE) later brought forward the belief that everything was made from air rather water; then Empedocles, in the 5th Century BC, proposed it was not one substance but four elements of water, air, earth and fire; and this progressed to Leucippus and Democritus (C.400 BCE) concluding that cosmos is made up of atoms and empty space, which is a lot more recognizable to modern understandings of metaphysics, and led to the knowledge of atoms today.
Thales was also well known for his geometry, his understanding was practical and theoretical, saying things like, ”Space is the greatest thing, as it contains all things”. Thales is known amongst mathematicians for “Thales’ Theorem”; although there are actually two Theorems attributed to Thales, one being also known as the “Intercept theorem”, the other describing a triangle which is inscribed within a circle, having the circle’s diameter as one length. Although later Eudemus also attributed other mathematical reasonings to Thales too, many of which are quite vital to human understanding of the world.
For further reading on Thales, I would recommend reading Aristotle’s Metaphysics, or G.E.R. Lloyd’s Early Greek Science: Thales to Aristotle.
Metaphysics: - The branch of philosophy concerned with explaining the fundamental nature of being and the world.
Cosmological: - the philosophical study of the origin and nature of the universe.
G. E. R. Lloyd. Early Greek Science: Thales to Aristotle
Kirk, G.S.; Raven, J.E. (1957). The Presocratic Philosophers