Democritus was a Greek philosopher who lived between 460-370 BCE. He was a contemporary of Socrates and a member of the Milesian school of philosophy. He is best known for his development of the atomic theory of matter, which proposed that the universe is made up of indestructible, indivisible particles called atoms.

Democritus argued that everything in the observable universe, including living organisms, can be explained by the properties and interactions of atoms. He also believed that the universe was infinite and eternal, and that the different qualities of matter were a result of the shape, size, and arrangement of atoms.

Democritus' atomic theory was not widely accepted during his lifetime, but it was later revived and developed by Epicurus and Lucretius. His ideas on atomic theory laid the foundations for the development of modern physics, chemistry and atomic theory. He also wrote on ethics, politics, and epistemology, but his works did not survive. He was known as the "laughing philosopher" because of his emphasis on the pursuit of happiness and avoidance of excess.