Empedocles answered the Parmenidean difficulty, that what is cannot come from what is not, by denying that anything comes to be at all. There are four roots, fire, air, earth and water, which are ungenerated and indestructible, and there are two forces, Love which draws them together and Strife which drives them apart. What people call birth and death is only mixing and separating, and the appearance of new things is the appearance of new proportions. Bone is so much fire to so much earth and water, in a stated ratio; the arrangement is quantitative and he gives numbers.
The cosmos runs on a cycle between the two forces. At the extreme of Love everything is blended into a motionless sphere in which no individual thing exists; Strife enters and separates, and in one of the stages of that separation limbs and organs wander alone and join at random, so that ox headed men and creatures with faces on both sides appear, and only the combinations that can survive and reproduce persist. Some readers have called that an anticipation of natural selection and others have thought the resemblance superficial. The four roots themselves had the longer career: they pass through Aristotle into medieval medicine, alchemy, and eventually into the elemental scheme that modern occult and divinatory systems still use.