These two propositions are far from being the same: "I have found that such an object has always been attended with such an effect," and: "I foresee that other objects, which are in appearance similar, will be attended with similar effects." I shall allow, if you please, that the one proposition may justly be inferred from the other; I know, in fact, that it always is inferred. But you must confess that the inference is not intuitive; neither is it demonstrative. Of what nature is it, then? To say it is experimental is begging the question. For all inferences from experience suppose, as their foundation, that the future will resemble the past, and that similar powers will be conjoined with similar sensible qualities. If there be any suspicion that the course of nature may change, and that the past may be no rule for the future, all experience becomes useless, and can give rise to no inference or conclusion. It is impossible, therefore, that any arguments from experience can prove this resemblance of the past to the future, since all these arguments are founded on the supposition of that resemblance. Let the course of things be allowed hitherto ever so regular, that alone, without some new argument or inference, proves not that for the future it will continue so. In vain do you pretend to have learned the nature of bodies from your past experience. Their secret nature, and consequently all their effects and influence, may change without any change in their sensible qualities. This happens sometimes, and with regard to some objects. Why may it not happen always, and with regard to all objects? What logic, what process of argument, secures you against this supposition? My practice, you say, refutes my doubts. But you mistake the purport of my question. As an agent, I am quite satisfied on the point; but as a philosopher, who has some share of curiosity, I will not say scepticism, I want to learn the foundation of this inference. All inferences from experience are effects of custom, not of reasoning. We have already observed that nature has established connections among particular ideas, and that no sooner one idea occurs to our thoughts than it introduces its correlative, and carries our attention towards it by a gentle and insensible movement. These principles of connection or association we have reduced to three--namely, _resemblance_, _contiguity_, and _causation_, which are the only bonds that unite our thoughts together and beget that regular train of reflection or discourse which, in a greater or less degree, takes place among mankind.