To take a silly example, say your friend sits across a table from you and quickly drinks a pint of beer. He then becomes red in the face and stumbles around drunk. This is a clear case of causation if anything is: his drinking causes his redness and stumbling. But although you may perceive his drinking and perceive as well his subsequent redness and stumbling, do you perceive the causal link between the drinking and the redness and stumbling? What would that even look like? But then if Hume is right and all of our ideas are built up out of simpler ideas copied from sense impressions we’ve had, then how is it possible for us to ever have the idea of the causal relation linking the drinking and the redness and stumbling?