The Four Causes

To know a thing, Aristotle held, is to know its cause — its aitia (αἰτία), the 'reason why' it is as it is. He argued that there are four such causes, four distinct answers to the question 'why?', and that a complete explanation gives all four.

The fourfold why

The material cause is that out of which a thing is made — the bronze of a statue, the matter underlying it. The formal cause is its form or essence, the shape and definition that make it what it is. The efficient cause is the source of motion or change — the sculptor who works the bronze. And the final cause is its telos, 'that for the sake of which' it exists — the end the statue serves, the function the eye performs.

Nature and purpose

Aristotle's deepest claim is that nature itself works for ends: the final cause is primary, for matter and motion serve the form a thing strives to realize. This teleological vision — that things have built-in purposes, that nature does nothing in vain — governed natural philosophy for two thousand years, and remains the great alternative to a purely mechanical picture of the world.