The ancient atomists — Democritus and Leucippus — explained natural phenomena by the chance collisions of material particles. There is no purpose, no direction, no end in nature: things are as they are because the atoms happened to arrange themselves this way. Aristotle takes this view seriously but finds it inadequate. It cannot explain the regularity of natural development: why do acorns always become oaks and never elms, if only chance is at work? The reliability of natural processes requires a principle of direction internal to the things themselves.
Aristotle's teleology is not the theology of an external designer. Nature does not pursue ends because a craftsman designed it to; it pursues ends because the natures of things are themselves end-directed. The form of a living organism just is its characteristic mode of development toward a mature actuality. This is what distinguishes the natural from the artificial: artefacts receive their form from an external agent; natural things carry the principle of their own development within themselves. Natural teleology is immanent, not imposed.
Aristotle does not deny that material necessity plays a role in natural processes — only that it suffices for explanation. A saw must be made of iron (material necessity), but this necessity is in the service of the end for which the saw exists (cutting). Similarly, the bones of animals must be hard, but this necessity exists because hardness is required for the function bones serve. The teleological and the necessary are not opposites but collaborative factors: to understand nature fully is to see how material necessity serves the ends that natural forms are oriented toward.
Natural teleology is defended in Physics II.8–9 and illustrated throughout Aristotle's biological works — especially Parts of Animals and Generation of Animals. His argument that "nature does nothing in vain" became a maxim of medieval natural philosophy and remained influential until the Scientific Revolution.
