Simplify the following text for a Grade 14 native English-speaking person.
Original: Leucippus or Leukippos ( Greek : , first half of 5th century BCE ) was one of the earliest Greeks to develop the theory of atomism â '' the idea that everything is composed entirely of various imperishable , indivisible elements called atoms â '' which was elaborated in greater detail by his pupil and successor , Democritus .
Simplified: Leucippus or Leukippos ( Greek : , first half of 5th century BC ) was among the earliest philosophers of atomism , the idea that everything is made up of many imperishable , indivisible elements called atoms .