Hume on Causation, Induction, and Custom

Hume's treatment of causation, developed at length in Book I, Part III of the Treatise and restated in Section VII of the Enquiry, begins from an application of the copy principle: we regularly speak and think as though causes are necessarily connected to their effects, yet careful examination of any particular instance of causation — one billiard ball striking another and setting it in motion — reveals to observation only the constant conjunction of two distinct events (contact, followed by motion), their contiguity in space and time, and their priority in temporal order, but never any additional, directly perceivable impression of a "necessary connection" binding the cause to the effect, such that the effect must follow given the cause. Since no single instance of a cause-effect pair yields an impression of necessary connection, Hume concludes that our idea of causal necessity, if it is to have any legitimate content at all, cannot be derived from any single observed instance, but must instead arise only after repeated observation of many similar constantly conjoined pairs.