All this, and everything else, which I believe, are nothing but ideas; though by their force and settled order, arising from custom and the relation of cause and effect, they distinguish themselves from the other ideas, which are merely the offspring of the imagination.
As to the influence of contiguity and resemblance, we may observe, that if the contiguous and resembling object be comprehended in this system of realities, there is no doubt but these two relations will assist that of cause and effect, and infix the related idea with more force in the imagination.

This I shall enlarge upon presently.

Mean while I shall carry my observation a step farther, and assert, that even where the related object is but feigned, the relation will serve to enliven the idea, and encrease its influence.

A poet, no doubt, will be the better able to form a strong description of the Elysian fields, that he prompts his imagination by the view of a beautiful meadow or garden; as at another time he may by his fancy place himself in the midst of these fabulous regions, that by the feigned contiguity he may enliven his imagination.
But though I cannot altogether exclude the relations of resemblance and contiguity from operating on the fancy in this manner, it is observable that, when single, their influence is very feeble and uncertain.

As the relation of cause and effect is requisite to persuade us of any real existence, so is this persuasion requisite to give force to these other relations.