Anaximander was a philosopher from Miletus in ancient Ionia, active in the sixth century BCE. He is considered one of the earliest Greek thinkers and a central figure among the Milesian school, following Thales and teaching Anaximenes. The traditional dates for his life, approximately 610–546 BCE, are derived from later sources and are not certain.

He is credited with major innovations in both thought and practical science. According to modern scholars, he was likely the first Greek to write a prose treatise on philosophy, often referred to by the title On Nature. He was also a pioneering geographer, creating one of the first known maps of the inhabited world, and is said to have introduced the sundial to Greece. His written works are lost, surviving only through references and a single brief quotation in later authors. The exact number and titles of his works, which may have included a geographical study, are disputed due to this fragmentary evidence.

Anaximander’s historical importance is foundational. His primary philosophical contribution was proposing the apeiron (the boundless or indefinite) as the fundamental origin of all things, moving beyond the idea of a single material element like water. He developed an early cosmological model where the Earth rests freely at the center, and he speculated about multiple worlds undergoing cycles of creation and destruction. He also offered a pioneering biological idea, suggesting humans originally developed from fish-like creatures. This combination of bold metaphysical speculation with detailed theories about the natural world set a pattern for early Greek science and philosophy.