 Hume now moves to the only remaining possibility. If causal inferences don’t involve a priori reasoning about relations of ideas, they must concern matters of fact and experience. When we’ve had many experiences of one kind of event constantly conjoined with another, we begin to think of them as cause and effect and infer the one from the other. But even after we’ve had many experiences of a cause conjoined with its effect, our inferences aren’t determined by reason or any other operation of the understanding.