Hume really gave two definitions, not one, the first of regularity (i.e., the cause is regularly followed by the effect) and the second of the counterfactual (“if the first object had not been …”). While philosophers and scientists had mostly paid attention to the regularity definition, Lewis argued that the counterfactual definition aligns more closely with human intuition: “We think of a cause as something that makes a difference, and the difference it makes must be a difference from what would have happened without it.”
