But why need we seek for other arguments to prove, that a present impression with a relation or transition of the fancy may enliven any idea, when this very instance of our reasonings from cause and effect will alone suffice to that purpose? 'Tis certain we must have an idea of every matter of fact which we believe. 'Tis certain that this idea arises only from a relation to a present impression. 'Tis certain that the belief superadds nothing to the idea, but only changes our manner of conceiving it, and renders it more strong and lively. The present conclusion concerning the influence of relation is the immediate consequence of all these steps; and every step appears to me sure and infallible. 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 'tis 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. 'Tis 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 'tis 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. I find that an impression, from which, on its first appearance, I can draw no conclusion, may afterwards become the foundation of belief, when I have had experience of its usual consequences. We must in every case have observed the same impression in past instances, and have found it to be constantly conjoined with some other impression. This is confirmed by such a multitude of experiments, that it admits not of the smallest doubt.