"Empedocles... changed... [Greek philosophy] from monism to a kind of pluralism. To avoid the difficulty that one primary substance cannot explain the variety of things and events, he assumed four basic elements, earth, water, air and fire.... mixed... and separated by... Love and Strife. Therefore, these latter two... are responsible for the imperishable change. Empedocles describes the formation of the world... First, ...the infinite Sphere of the One, as in the philosophy of Parmenides. But in the primary substance all the four "roots" are mixed... by Love. Then, when Love is passing out and Strife coming in, the elements are partially separated and partially combined. After that the elements are completely separated and Love is outside the World. Finally, Love is bringing the elements together again and Strife is passing out, so... we return to the original Sphere. This doctrine... represents... a more materialistic view... Here for the first time... a few [fundamentally different] substances... explains the infinite variety of things and events. Pluralism never appeals to those who are wont to think in fundamental principles. But it is a reasonable... compromise, which avoids the difficulty of monism and allows... some order."