Philosophy (General).

Democritus suggested that all substance in the universe was made up of particles so minute that nothing smaller was possible. He regarded those particles (atoms) as unchangeable and indestructible, and as the only content of the universe besides the very space in which they existed.

William James believed that every difference in thinking must make a difference to someone, somewhere. If two theories differ, the difference becomes clear when we know (1) how they differ over what the facts are, and (2) the difference in our behavior if we believe that one or the other is true.