The Pre-Socratic Philosophers are defined as the Greek thinkers who developed independent and original schools of thought from the time of Thales of Miletus (l. c. 585 BCE) to that of Socrates of Athens (470/469-399 BCE). They are known as Pre-Socratics because they pre-date Socrates.
Thales of Miletus initiated the intellectual movement that produced the works now known as ancient Greek philosophy by inquiring into the First Cause of existence, the matter from which all else came, which was also the causative factor in its becoming. He concluded that water was the First Cause because it could assume different forms (steam when heated, ice when frozen) and seemed to inform all living things.
This conclusion was rejected by later philosophers beginning with Anaximander (l. c. 610 – c. 546 BCE) who argued that the First Cause was beyond matter and was, in fact, a cosmic force of creative energy constantly making, destroying, and remaking the observable world. The philosophers who followed these two all established their own schools of thought with their own concepts of a First Cause, steadily building on the accomplishments of predecessors until philosophy found full expression and depth in the works of Plato (l. 424/423-348/347 BCE), who attributed his own ideas to the figure of Socrates.