  But what is it? To answer this question, we must determine from what sensory experiences we derive the idea of causation. It turns out, as Reid reads Hume, that the sensory experiences that give rise to our idea of causation are sensory experiences of what Hume calls constant conjunction. The heating of the water is regularly followed by the water's boiling. To say that the one event causes the other is just to say that the two events always co-occur, or that there is a brute law linking them. This is not to say that the first event necessitates the second. The necessitation that we ordinarily take to be involved in causation is not in the world; it is in our heads as an expectation arising due to previous experience.