The Greek word Aristotle uses for cause (aitia) is better translated as "explanation" or "reason why." When we ask why something is the case, we can mean several different things. Why is this a statue? Because it is made of bronze (material cause). Because it has this human shape (formal cause). Because a sculptor fashioned it (efficient cause). Because it was commissioned to honour a god (final cause). A complete explanation of any phenomenon must address all four questions; a partial answer, however accurate, leaves the understanding incomplete.
The most controversial element of Aristotle's framework is the final cause — the for-the-sake-of-which that governs natural processes. In artefacts, the final cause is obvious: a house exists for the purpose of shelter. Aristotle extends this teleological structure to nature itself. The swallow builds its nest not because it was designed by an intelligent agent but because shelter-seeking is part of what it is to be a swallow — the goal is internal to the organism's nature. Nature acts for an end, not randomly, even without deliberation.
Of the four causes, form and final cause often coincide: the nature of a thing — what it is to be that kind of thing — is also what that thing is for. The form of a knife is its cutting edge; the end of the knife is cutting; these are the same thing described differently. This convergence gives Aristotle's explanation of nature its characteristic shape: to understand a natural thing is to understand what it is striving to become, the fully actualised form it tends toward when it develops normally. Final causation is not imposed from outside but immanent in the form itself.
The four causes are introduced in Physics II.3 and in Metaphysics V.2. Aristotle acknowledges his predecessors — the Pre-Socratics grasped material causes; Plato grasped formal causes — but argues none had a systematic account of all four. Final causation in nature (Physics II.8) remains one of his most discussed and contested doctrines.
