On the other hand, did every idea influence our actions, our condition would not be much mended.

For such is the unsteadiness and activity of thought, that the images of every thing, especially of goods and evils, are always wandering in the mind; and were it moved by every idle conception of this kind, it would never enjoy a moment's peace and tranquillity.
Nature has, therefore, chosen a medium, and has neither bestowed on every idea of good and evil the power of actuating the will, nor yet has entirely excluded them from this influence.

Though an idle fiction has no efficacy, yet we find by experience, that the ideas of those objects, which we believe either are or will be existent, produce in a lesser degree the same effect with those impressions, which are immediately present to the senses and perception.

The effect, then, of belief is to raise up a simple idea to an equality with our impressions, and bestow on it a like influence on the passions.

This effect it can only have by making an idea approach an impression in force and vivacity.