Leucippus was a Greek philosopher of the fifth century BCE. He is regarded as the founder of atomism, a school of thought later developed by his more famous pupil, Democritus. Very little is known with certainty about his life. His birthplace is uncertain, with ancient sources suggesting Miletus, Elea, or Abdera. According to modern scholars, he was likely active around 440–430 BCE. Some ancient accounts propose he was a student of the Eleatic philosophers, which would explain how his theories formed a direct response to theirs.

He is credited with two major works, The Great World-System and On Mind, but both are completely lost. They survive only through references and quotations in later writers. The attribution of The Great World-System was disputed even in antiquity, with some assigning it to Democritus. A single, important quote from On Mind is preserved by Aristotle, stating that nothing happens randomly, but everything occurs for a reason and by necessity.

Leucippus’s historical importance lies in originating the core ideas of atomist philosophy. He proposed that the universe is composed of an infinite number of tiny, indivisible, and eternal particles called atoms, moving in empty space or void. This was a materialistic and mechanistic system designed to explain change, diversity, and the natural world, directly challenging other philosophical schools of his time. Although his own writings are lost, ancient authorities like Aristotle credit him with founding this influential theory. His ideas were systematized by Democritus and later shaped Epicurean philosophy, leaving a lasting legacy on scientific and philosophical thought.