Anaximander (/əˌnæksɨˈmændər/; Greek: Ἀναξίμανδρος Anaximandros; c. 610 – c. 546 BC) was a pre-Socratic Greek philosopher who lived in Miletus, a city of Ionia; Milet in modern Turkey. He belonged to the Milesian school and learned the teachings of his master Thales. He succeeded Thales and became the second master of that school where he counted Anaximenes and arguably, Pythagoras amongst his pupils.

Little of his life and work is known today. According to available historical documents, he is the first philosopher known to have written down his studies,[1] although only one fragment of his work remains. Fragmentary testimonies found in documents after his death provide a portrait of the man.

Anaximander was one of the earliest Greek thinkers at the start of the Axial Age, the period from approximately 700 BC to 200 BC, during which similarly revolutionary thinking appeared in China, India, Iran, the Near East, and Ancient Greece. He was an early proponent of science and tried to observe and explain different aspects of the universe, with a particular interest in its origins, claiming that nature is ruled by laws, just like human societies, and anything that disturbs the balance of nature does not last long.[2]Like many thinkers of his time, Anaximander's contributions to philosophy relate to many disciplines. In astronomy, he tried to describe the mechanics of celestial bodies in relation to the Earth. In physics, his postulation that the indefinite (or apeiron) was the source of all things led Greek philosophy to a new level of conceptual abstraction. His knowledge ofgeometry allowed him to introduce the gnomon in Greece. He created a map of the world that contributed greatly to the advancement of geography. He was also involved in thepolitics of Miletus and was sent as a leader to one of its colonies.

Anaximander claimed that an "indefinite" (apeiron) principle gives rise to all natural phenomena. Carl Sagan claims that he conducted the earliest recorded scientific experiment.[3]