The tradition of Greek philosophy is one of the most influential in the intellectual history of the world. It began as an effort to understand the world through reason and observation. The so-called pre-Socratic philosophers inaugurated the philosophical tradition. Their main concern was the external world, its composition and structure, and there was no distinction between “philosophy” and “science” such as would later emerge. Thales, the man often identified as the “first philosopher,” suggested in the sixth century BCE that the fundamental element of the world was water, while Anaximenes later in the century suggested that it was air.