Anaximander of Miletus was thought to be a pupil of Thales and teacher of Anaximenes. Together they formed what we call the Milesian School of Pre-Socratic philosophy. He is credited with inventing the gnomon on the sundial and with drawing a map of the world in which people live. Like his teacher, he believed in an arche (principle) that was at the basis of all the universe, but for him, it was an indefinite nature apeiron. All things came out of the apeiron through a process using heat and cold. Anaximander may have been the first to write a philosophical treatise, and in prose. He is said to have created the investigation of nature, according to Charles H. Kahn in Anaximander and the Origins of Greek Cosmology.
One fragment of his writing remains which says: They pay penalty and retribution to each other for their injustice in accordance with the ordering of time. This is an important fragment, since it is the only Milesian school primary source material (Kahn), although it comes to us via Simplicius (6th century A.D.) repeating what Theophrastus (successor of Aristotle) had written about the Milesians.
We have first hand material of the Pre-socratics only from the end of the sixth century/start of the fifth B.C. Even then, material is spotty. So our knowledge of the Pre-Socratic philosophers comes from fragments of their works included in the writing of others. The Presocratic Philosophers: A Critical History with a Selection of Texts, by G.S. Kirk and J.E. Raven provides these fragments in English. Diogenes Laertius provides biographies of the Pre-Socratic philosophers: Lives of Eminent Philosophers, Loeb Classical Library.Also see: Notable Ancient People
Main Source: Curd, Patricia, "Presocratic Philosophy", The Stanford Encyclopedia of Philosophy, Edward N. Zalta (ed.).