Democritus was a Greek philosopher born around 460-457 BC in Abdera, Thrace. He developed the first atomic theory, proposing that all matter is made up of indivisible atoms moving through empty space. Democritus believed that atoms were the fundamental building blocks of the natural world and that their behavior determined natural phenomena. He and his mentor Leucippus are considered the founders of atomic theory. Democritus was highly respected in his lifetime for making discoveries and predictions that were later proven true.