To convince us that all the laws of nature, and all the operations of bodies without exception, are known only by experience, the following reflections may perhaps suffice. Were any object presented to us, and were we required to pronounce concerning the effect which will result from it, without consulting past observation, after what manner, I beseech you, must the mind proceed in this operation? It must invent or imagine some event, which it ascribes to the object as its effect; and it is plain that this invention must be entirely arbitrary. The mind can never possibly find the effect in the supposed cause by the most accurate scrutiny and examination. For the effect is totally different from the cause, and, consequently, can never be discovered in it.
A stone or piece of metal raised into the air and left without any support immediately falls. But, to consider the matter a priori, is there anything we discover in this situation which can beget the idea of a downward rather than an upward, or any other motion, in the stone or metal?
In a word, then, every effect is a distinct event from its cause. It could not, therefore, be discovered in the cause, and the first invention or conception of it, a priori, must be entirely arbitrary. And, even after it is suggested, the conjunction of it with the cause must appear equally arbitrary, since there are always many other effects which to reason must seem fully as consistent and natural. In vain, therefore, should we pretend to determine any single event, or infer any cause or effect, without the assistance of observation and experience.
Hence, we may discover the reason why no philosopher who is rational and modest has ever pretended to assign the ultimate cause of any natural operation, or to show distinctly the action of that power which produces any single effect in the universe.
I say, then, that even after we have experience of the operations of cause and effect, our conclusions from that experience are not founded on reasoning, or any process of the understanding.
The bread which I formerly ate nourished me; that is, a body of such sensible qualities was at that time endued with such secret powers; but does it follow that other bread must also nourish me at another time, and that like sensible qualities must always be attended with like secret powers? The consequence seems nowise necessary. At least, it must be acknowledged that there is here a consequence drawn by the mind, that there is a certain step taken; a process of thought, and an inference which wants to be explained.
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.