Aristotle, answered from the texts and cited to the page.
Knowledge is the object of inquiry, and we do not think we know a thing until we have grasped its why — its primary cause.1 Since causes are spoken of in several senses, when one inquires into the cause of something one should state all the possible causes.2 The first is the material cause: that out of which a thing comes to be and which persists through the change.
The bronze of a statue, the silver of a bowl — these are material causes, along with the genera under which such materials fall.3 The second is the formal cause: the form or archetype, the statement of the essence. Why is the angle in a semicircle a right angle? Because it is half of two right angles — and that definition just is what the right angle is.
The formal cause and the definitory formula are, in such cases, identical.4 The third is the efficient cause: the primary source of change or of rest. The father is a cause of the child; the adviser is a cause of the action; the maker is a cause of the thing made.5 Why did the Athenians become involved in the Persian war? Because they raided Sardis with the Eretrians — that act originated the war, and so stands as the efficient cause in the middle term of the demonstration.6
The fourth is the final cause: the end, that for the sake of which a thing is done. Health is the cause of walking; the preservation of one's goods is the cause of a house. When we say why someone walks and receive the answer "to be healthy," we think we have given the cause — and we are right to think so.7 All the intermediate steps, whether drugs or instruments or purgings, are also for the sake of the end, though they differ in that some are instruments and others are actions.8
Now, several consequences follow from this fourfold structure. First, the same thing can have several causes simultaneously, and in no accidental sense: both the art of sculpture and the bronze are causes of the statue, though not in the same way — one as matter, the other as source of movement.9 Second, things can be causes of one another: exercise causes good condition, and good condition causes exercise, though one is cause as end and the other as source of movement.10
Third, not every cause applies to every kind of being: an eclipse has no material cause in the relevant sense — the moon is simply what undergoes it — and some eternal things have no matter of the generable sort at all.11 When we inquire into man, we must state all four: the menstrual fluid as material cause, the seed as efficient cause, the essence as formal cause, and the end as final cause — and it may be that the formal and final causes here coincide.12
The proximate causes are what we must name, not remote ones: not fire or earth as matter, but the matter peculiar to the thing in question.13 The good and the beautiful are themselves beginnings — of knowledge and of movement in many things — and the final cause belongs among them.14 An account that omits any of the four has not finished its work.
Knowledge is the object of our inquiry, and men do not think they know a thing till they have grasped the 'why' of (which is to grasp its primary cause).Physics
When one inquires into the cause of something, one should, since 'causes' are spoken of in several senses, state all the possible causes.Metaphysics
that out of which a thing comes to be and which persists, is called 'cause', e.g. the bronze of the statue, the silver of the bowl, and the genera of which the bronze and the silver are species.Physics
Why is the angle in a semicircle a right angle?...B is the half of two right angles...Moreover, B is identical with (b) the defining form of A, since it is what A's definition signifies.Posterior Analytics
the primary source of the change or coming to rest; e.g. the man who gave advice is a cause, the father is cause of the child, and generally what makes of what is made and what causes change of what is changed.Physics
'Why did the Athenians become involved in the Persian war?' means 'What cause originated the waging of war against the Athenians?' and the answer is, 'Because they raided Sardis with the Eretrians', since this originated the war.Posterior Analytics
The end, i.e. that for the sake of which a thing is; e.g. health is the cause of walking. For 'Why does one walk?' we say; 'that one may be healthy'; and in speaking thus we think we have given the cause.Metaphysics
The same is true also of all the intermediate steps which are brought about through the action of something else as means towards the end, e.g. reduction of flesh, purging, drugs, or surgical instruments are means towards health. All these things are 'for the sake of' the end, though they differ from one another in that some are activities, others instruments.Physics
both the art of sculpture and the bronze are causes of the statue not in respect of anything else but qua statue; not, however, in the same way, but the one as matter and the other as source of the movement.Metaphysics
things can be causes of one another (e.g. exercise of good condition, and the latter of exercise; not, however, in the same way, but the one as end and the other as source of movement).Metaphysics
what is the cause of eclipse? What is its matter? There is none; the moon is that which suffers eclipse. What is the moving cause which extinguished the light? The earth.Metaphysics
What is the material cause? Shall we say 'the menstrual fluid'? What is moving cause? Shall we say 'the seed'? The formal cause? His essence. The final cause? His end. But perhaps the latter two are the same.Metaphysics
It is the proximate causes we must state. What is the material cause? We must name not fire or earth, but the matter peculiar to the thing.Metaphysics
the final cause-for the good and the beautiful are the beginning both of the knowledge and of the movement of many things.Metaphysics