Anaxagoras was a Greek philosopher from Clazomenae in Ionia, active in the fifth century BCE. He is a major figure among the Presocratic thinkers. He moved to Athens around the middle of the century, becoming the first significant philosopher to live and teach there. His presence influenced the city's intellectual culture, including the statesman Pericles. His scientific theories about the heavens, which described the sun as a hot mass of metal and the moon as earthy rock, led to his prosecution and conviction for impiety. He avoided execution and spent his final years in exile.

He is known to have written a single book, commonly titled On Nature. Only fragments of this work survive, quoted by later ancient authors. His philosophy is famous for two core ideas. First, he argued that in the beginning, all things were mixed together, and a cosmic intelligence he called Nous (Mind) set this mixture into a spinning motion to form the ordered world. Second, he proposed that nothing truly comes into being or is destroyed, but only mixes and separates. According to his view, every substance contains a portion of every other kind of matter.

Modern scholars see Anaxagoras as a pivotal thinker for introducing a non-material, ordering principle like Nous into physical speculation. His trial also made him an early symbol of the tension between philosophical inquiry and traditional religious belief.