One billiard ball strikes another, and the second moves. We say the first caused the second. Causality is the relation between events where one brings about another. It is so basic to how we think that it is hard to imagine doing without it. Yet philosophers have struggled for centuries to say exactly what the relation consists in.

David Hume argued that we never perceive causation directly. We see one event followed by another, repeatedly, and the mind develops an expectation. Causation, on his account, is constant conjunction plus a habit of expectation. Kant rejected that, arguing that causality is a category the mind imposes on experience. Without it, we could not have coherent experience at all. In the 20th century, philosophers distinguished different kinds of causes: necessary and sufficient, proximate and distant, contributory and decisive. Scientists use probabilistic and counterfactual accounts: a cause is something that makes a difference to the probability or occurrence of an effect. Correlation is not causation, as the slogan goes, but determining when it is requires care.

Accounts of causation

• Regularity. Causation is constant conjunction.
• Counterfactual. A cause is something without which the effect would not occur.
• Probabilistic. A cause raises the probability of an effect.
• Mechanistic. A cause produces an effect through a mechanism.

Causality matters in law, medicine, and science, where attributing effects to causes has practical consequences.