Commonsense thinking tells you that there are cause-effect relationships in the world. You are apt to accept statements like “Every event must have a cause.” But Hume goes beyond common sense and asks, “What is the origin of the idea of causality?” Since ideas are copies of impressions, he asks, what impression gives you the idea of causality? His answer is that there is no impression corresponding to that idea.

The games of billiards can be used to illustrate this principle. If you see event A, a seven ball struck by a cue and then event B, the seven ball contacting a six ball, you have witnessed two events. You have an impression of each event. But Hume says there is no third event or relationship like causality.

How then does the idea of causality arise in the mind? It arises in two ways. Experience furnishes you with two relations: first, there is the relation of contiguity, for A and B are always close together; second, there is priority in time, for A, the “cause” always precedes B, “the effect.” But there is still another relation that the idea of causality suggests to commonsense observers. You think that between A and B there is a “necessary connection.” But neither contiguity nor priority implies “necessary” connection between objects.

All that this necessary connection boils down to is the habit of mind of expecting event B whenever you have seen event A. “It is by experience only that we infer the existence of one object from another,” he says. You have seen one event constantly conjoined with a second. But this constant conjunction of objects is not a quality in the objects you observe but is rather a “habit of association” in the mind produced by the repetition of instances of A and B.

Hume's answer is that you are continually observing pairs of events, such as cues striking balls, flames producing heat, penicillin curing strep throat, and so on. One part of the pair makes you think of the other: you come to expect heat when you see flames. In the end you come to say that a flame must produce heat and you call the flame the cause of the heat. This is where the idea of cause and effect comes from. What you call cause and effect is not “in the world” but “in us”; it is psychological predisposition to expect a second event to occur after seeing the first. What Hume has provided is a description of the way in which the belief about cause and effect arises.