 And though [one] should be convinced, that his understanding has no part in the operation, he would nonetheless continue in the same course of thinking. There is some other principle, which determines him to form such a conclusion. This principle is CUSTOM or HABIT. For wherever the repetition of any particular act or operation produces a propensity to renew the same act or operation, without being impelled by any reasoning or process of the understanding; we always say, that this propensity is the effect of Custom. By employing that word, we pretend not to have given the ultimate reason of such a propensity. We only point out a principle of human nature, which is universally acknowledged, and which is well known by its  effects.[7]