Atomism (from ancient Greek atomos, meaning "uncuttable") is a natural philosophy that developed in several ancient traditions. The atomists theorized that the natural world consists of two fundamental parts: indivisible atoms and empty void.

According to Aristotle, atoms are indestructible and immutable and there are an infinite variety of shapes and sizes. They move through the void, bouncing off each other, sometimes becoming hooked with one or more others to form a cluster. Clusters of different shapes, arrangements, and positions give rise to the various macroscopic substances in the world.

According to Aristotle, the heavenly bodies are the most perfect realities, (or "substances"), whose motions are ruled by principles other than those of bodies in the sublunary sphere. The latter are composed of one or all of the four classical elements (earth, water, air, fire) and are perishable; but the matter the heavens are made of is imperishable aether, so they are not subject to generation and corruption. Hence their motions are eternal and perfect, and the perfect motion is the circular one, which, unlike the earthly up-and down-ward locomotions, can last eternally selfsame. As substances, celestial bodies have matter (aether) and a form: it seems that Aristotle did regard them as living beings with a rational soul as their form