Philosophical treatise: On Nature

Overview
Anaxagoras’ On Nature sets out a bold pluralist cosmology that preserves Parmenides’ ban on absolute coming-to-be and passing-away while explaining the world’s variety. It survives in fragments quoted by later writers, but its core theses are clear: all things are mixtures of infinitely fine ingredients; alteration is a change in predominance rather than creation ex nihilo; and a unique, unmixed principle called Nous (Mind) initiates and orders cosmic motion.

Mixture and No Coming-to-Be
The treatise opens from the Eleatic insight that what-is cannot arise from what-is-not. To reconcile this with evident change, Anaxagoras holds that “in everything there is a portion of everything.” There is no smallest: the constituents are indefinitely divisible, and no qualitative element is ever entirely absent from any portion of matter. Change is the emergence of a dominant ingredient within a persistent mixture. Nourishment illustrates the point: bread makes bone not by producing bone from nothing, but because bone-like ingredients latent in the bread become predominant through separation and reconfiguration. Aristotle later called these basic stuffs “homoeomeries” (flesh-in-flesh, bone-in-bone), and Anaxagoras also speaks of “seeds,” emphasizing the generative capacities distributed throughout matter.

Nous (Mind)
Distinct from all mixed things stands Nous, described as the finest, purest, and unmixed. It is infinite in power and knowledge, self-ruling, and it knows and orders all. Because Nous is unalloyed, it can act upon the mixture without being impeded or diluted by it. This Mind initiates a rotational motion that begins in a small region and spreads, arranging and separating what had been indiscriminately “together.” Although Anaxagoras credits Nous with ordering, he typically explains outcomes by physical features, density, speed, position, rather than by purposes, a point later remarked upon by Plato.

Cosmos and Astronomical Accounts
Originally, all ingredients were together “in a heap.” The rotation set off by Nous sorts them by fineness and weight: the rare and light are flung outward into ether and fire; the dense and moist gather toward the center as air, clouds, water, earth. The heavenly bodies form from fiery rings torn from the rotation, becoming sun, moon, and stars. The sun is a red-hot stone larger than the Peloponnese; the moon is earthy and receives its light from the sun, so phases and eclipses follow from its positions and shadows. The Milky Way is the combined light of distant stars obstructed by our viewpoint. Winds, rain, and meteors receive natural explanations in terms of heat, cold, and the motion of the mixture. Earth remains suspended amid the moving air and ether, not supported by a mythic pillar but stabilized within the rotational system.

Life, Perception, and Knowledge
Living things share in Nous to differing degrees, which accounts for their capacities for motion and cognition. Sense perception relies on contrasts within mixed organs encountering mixed external objects; perception arises when a contrary predominates and stands out against what is already present. Because everything contains everything, knowledge and error both reflect how mixtures reveal or obscure predominances, and intellect is required to discriminate what is hidden by likeness.

Method and Legacy
On Nature advances a consistently naturalistic picture: celestial and meteorological phenomena are explained by material mixtures, rotation, and separation rather than by divine whim. By pairing an all-pervasive plurality with an unmixed Mind, Anaxagoras preserves the Eleatic prohibition on creation from nothing while making room for ordered change. The result is a pivotal step from early monism toward later atomism and a template for scientific explanation that treats even the heavens as part of a single physical order.