Hume attacked the scientific and common sense idea that there was a necessary connection between cause and effect. He argued that no matter how closely we observe one billiard ball striking another, there is nothing in the action of the first ball that makes the response of the second inevitable. Only through experience do we learn relationships of cause and effect. To say that an event of type A causes an event of type B is to say no more than that, in the past, events of type A have always been followed by events of type B. This is Hume’s constant conjunction theory of causation. The mind relates causes to effects, and vice versa, based on past experience alone, which produces an association of the ideas of causes and effects with each other. For example, the idea of bread is associated with the idea of nourishment.