Ask why a chair is the way it is.

It is made of wood. It has that shape. A joiner made it. It is for sitting on.

All four are answers. All four are true at once, none is a rival to the others, and a person who knew only one of them would not understand the chair. Aristotle's claim is that this is general — that any thing or event admits four kinds of answer, and that explanation is incomplete until you have all of them.

They are not four kinds of cause

The word "cause" is the problem, and it has cost this doctrine more readers than any objection to it.

In modern usage a cause is what pushed — the earlier event that produced the later one. Only one of Aristotle's four is like that. His word, aitia, means something closer to what is responsible for, or what you cite when asked to account for a thing.

So read them as four kinds of because, and the doctrine stops sounding like a superseded physics and starts sounding like an obvious observation about explanation:

  • because of what it is made of — material
  • because of the form it has — formal
  • because of what brought it about — efficient
  • because of what it is for — final

Modern usage kept the third and quietly dropped the rest. Not because they were refuted; because a particular research program did not need them.

What each one is

Material. The bronze of the statue, the wood of the chair, the cells of the organism. Aristotle's point is not that stuff explains everything but that nothing is explained without it — the bronze constrains what the statue can be.

Formal. The shape, structure or arrangement that makes it this rather than a heap. The chemistry of a heart and the chemistry of its ingredients are identical; what differs is the organization, and the organization is what a heart is. See form.

Efficient. What brought it about — the joiner, the sculptor, the collision. The only one modern usage recognizes, and Aristotle thinks it is the thinnest of the four taken alone: the sculptor explains that the statue came to be, and nothing about what it is.

Final. What it is for. The chair for sitting, the eye for seeing, the acorn for the oak. This is the contested one, and the whole later argument about whether nature has purposes is an argument about it — which belongs to teleology and is treated there rather than here.

What it is actually for

The doctrine is not a theory of physics and is better read as a diagnostic: when an explanation feels thin, one of the four is missing.

A company's results explained only by what people did — efficient cause — with nothing about how the thing is structured or what it exists to do, is the commonest business version, and it is why such explanations never quite settle anything.

And the final cause is the one most often missing, because it is the one modern habits train you not to ask for. What is this for is a different question from how did it come about, and a system whose purpose nobody can state is not understood however well its mechanism is documented.

Aristotle's fuller machinery for the same ground is act and potency — the acorn is potentially the oak — and the two doctrines are meant to be read together.