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.  For where upon the appearance of an impression we not only feign another object, but likewise arbitrarily, and of our mere good-will and pleasure give it a particular relation to the impression, this can have but a small effect upon the mind; nor is there any reason, why, upon the return of the same impression, we should be determined to place the same object in the same relation to it.  There is no manner of necessity for the mind to feign any resembling and contiguous objects; and if it feigns such, there is as little necessity for it always to confine itself to the same, without any difference or variation.  And indeed such a fiction is founded on so little reason, that nothing but pure caprice can determine the mind to form it; and that principle being fluctuating and uncertain, it is impossible it can ever operate with any considerable degree of force and constancy.  The mind forsees and anticipates the change; and even from the very first instant feels the looseness of its actions, and the weak hold it has of its objects.  And as this imperfection is very sensible in every single instance, it still encreases by experience and observation, when we compare the several instances we may remember, and form a general rule against the reposing any assurance in those momentary glimpses of light, which arise in the imagination from a feigned resemblance and contiguity.

The relation of cause and effect has all the opposite advantages.  The objects it presents are fixt and unalterable.  The impressions of the memory never change in any considerable degree; and each impression draws along with it a precise idea, which takes its place in the imagination as something solid and real, certain and invariable.  The thought is always determined to pass from the impression to the idea, and from that particular impression to that particular idea, without any choice or hesitation.

But not content with removing this objection, I shall endeavour to extract from it a proof of the present doctrine.  Contiguity and resemblance have an effect much inferior to causation; but still have some effect, and augment the conviction of any opinion, and the vivacity of any conception.  If this can be proved in several new instances, beside what we have already observed, it will be allowed no inconsiderable argument, that belief is nothing but a lively idea related to a present impression.