The most important philosophers before Socrates (pre-Socratic) of the 6th-5th century B.C. are the first who attempted to explain rationally, with their logic, the genesis of the Universe, the Earth, as well as living beings, humanity, and its physiology and anatomy.

This book mentions the most significant among them, who came from the Greek colony of Asia Minor (Thales, Anaximander, Anaximenes), Miletus, the colonies of Southern Italy (Great Greece), Sicily, as well as from other places such as Athens, Thrace (Democritus), and Samos (Pythagoras).

It is noted that the pre-Socratic philosophers wrote books (on papyrus), in both prose and poetic form, of which, however, only small fragments have survived. These books, usually referred to as On Nature, are mentioned by later scholars (Simplicius, etc.), who are also the sources of information regarding their theories. We also have information from Plato, Aristotle, Plutarch, and others.