Metaphysics (1.4.6-1.4.10)

At any rate Love often differentiates and Strife combines: because whenever the universe is differentiated into its elements by Strife, fire and each of the other elements are agglomerated into a unity; and whenever they are all combined together again by Love, the particles of each element are necessarily again differentiated.

Empedocles, then, differed from his predecessors in that he first introduced the division of this cause, making the source of motion not one but two contrary forces.

Further, he was the first to maintain that the so-called material elements are four—not that he uses them as four, but as two only,

Such, then, as I say, is his account of the nature and number of the first principles.

Leucippus,[*] however, and his disciple Democritus[*] hold that the elements are the Full and the Void—calling the one what is and the other what is not. Of these they identify the full or solid with what is, and the void or rare with what is not (hence they hold that what is not is no less real than what is,[*] because Void is as real as Body); and they say that these are the material causes of things.

And just as those who make the underlying substance a unity generate all other things by means of its modifications, assuming rarity and density as first principles of these modifications, so these thinkers hold that the differences[*] are the causes of everything else.