Thales (625-546 B.C.)
While some say the same for Pythagoras, many scholars assert that Thales was the first philosopher.
The first of the Pre-Socratic philosophers, Thales was the first to ask questions about the structure and nature of the world. According to him, water was the basic element of everything. He also believed in the immortality of the soul.
Thales was also the first Greek to fix the length of the year at 365 days. He also predicted a solar eclipse in 585 B.C.