There enters nothing into this operation of the mind but a present impression, a lively idea, and a relation or association in the fancy betwixt the impression and idea; so that there can be no suspicion of mistake.
In order to put this whole affair in a fuller light, let us consider it as a question in natural philosophy, which we must determine by experience and observation.

I suppose there is an object presented, from which I draw a certain conclusion, and form to myself ideas, which I am said to believe or assent to.

Here it is evident, that however that object, which is present to my senses, and that other, whose existence I infer by reasoning, may be thought to influence each other by their particular powers or qualities; yet as the phenomenon of belief, which we at present examine, is merely internal, these powers and qualities, being entirely unknown, can have no hand in producing it.

It is the present impression, which is to be considered as the true and real cause of the idea, and of the belief which attends it.

We must therefore endeavour to discover by experiments the particular qualities, by which it is enabled to produce so extraordinary an effect.
First then I observe, that the present impression has not this effect by its own proper power and efficacy, and when considered alone, as a single perception, limited to the present moment.