The earliest known theories were developed in ancient India in the 6th century BC by Kanada, a Hindu Philosopher.  He statced about the basic units of matter and opined that all objects in the universe are made of these basic units.

Leucippus and Democritus, Greek philosophers in the 5th century BCE, presented their own theory of atoms.  The Greeks believed that atoms were all made of the same material but had different shapes and sizes, which determined the physical properties of the material.  In around 450 BCE, Democritus coined the term atomos, which meant "uncuttable".  Though both the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.