Causation is the relation by which one event brings about another. It is assumed by every explanation and has proved remarkably difficult to analyze.

Aristotle's four causes

Aristotle treated the question as asking why something is as it is, and distinguished four answers — the four causes. The material cause is what a thing is made of, the formal cause is its structure, the efficient cause is what produced it, and the final cause is what it is for. Modern usage has narrowed "cause" to the third of these, while the fourth survives as telos.

Hume's challenge

David Hume asked what is actually observed when one billiard ball strikes another. He found one event, then a second, and a constant conjunction between events of those kinds. He did not find necessity. The idea that the first made the second happen came from habit — the mind expecting the usual sequence — rather than from anything in the scene.

That analysis leaves causation as regular succession plus expectation, and it generates the problem of induction: if necessity is never observed, past regularities cannot guarantee future ones.

Modern repairs

Several strategies have been tried. Counterfactual accounts say that A caused B when B would not have occurred had A not occurred, which handles many cases and struggles with overdetermination — two assassins firing at once. Probabilistic accounts say causes raise the probability of their effects. Interventionist accounts, now prominent in statistics, define a cause as something that would change the effect if manipulated, which has the practical virtue of explaining why experiments are more informative than observation.

Causation remains one of the central topics of metaphysics and the hinge on which determinism turns.