The building components of matter are indivisible. Leucippus and Democritus wrote about this concept in the fifth century BCE. The Greeks termed these particles atomos, which means "indivisible," from whence the current word "atom" is derived. Democritus thought that atoms were uniform, solid, hard, incompressible, and indestructible, and that they travelled in an endless number of directions across empty space until they were stopped. The varied qualities of matter were defined by differences in atomic form and size. Atoms existed in Democritus' philosophy not just for substance, but also for attributes like perception and the human soul.