David Hume, answered from the texts and cited to the page.
The problem, stated plainly, is this: all our reasonings about cause and effect rest on experience, and all our reasonings from experience rest on the supposition that the course of nature will continue uniformly the same — that like causes in like circumstances will always produce like effects.1 But what determines us to form that supposition?
It cannot be reason. Adam himself, with all his science, could not demonstrate that the future must conform to the past; what is possible can never be demonstrated false, and a change in the course of nature is perfectly conceivable.2 Nor can any probable argument establish the conformity, because all probable arguments already presuppose it — to invoke experience as proof that experience will continue to hold is to beg the very question at issue.3
The difficulty runs deeper still. When I observe bread of a certain colour and consistence, I expect it to nourish me. But the colour and consistence are sensible qualities, while the power of nourishment is something secret, hidden behind what the senses deliver. Experience shows only that these qualities have hitherto been conjoined with these powers; it does not show that they must remain so.
The inference from past conjunction to future conjunction is neither intuitive nor demonstrative — and to call it experimental is simply to restate the problem, since every experimental inference already assumes that the future will resemble the past.4 If there be any suspicion that the course of nature may change, all experience becomes useless and can give rise to no inference or conclusion.5
Their secret nature, and consequently all their effects, may change without any change in their sensible qualities — it happens sometimes with some objects; why not always, with all?6 What, then, carries us across this gap? Not argument, but custom — the habit formed by repeated experience of conjunctions, which determines the mind to expect the same again.
When I see a billiard-ball moving toward another, my mind is immediately carried by habit to the usual effect, anticipating what the senses have not yet confirmed.7 The supposition that the future resembles the past is not founded on arguments of any kind; it is derived entirely from this habit, by which we are determined to expect for the future the same train of objects to which we have been accustomed.8
The conclusion, then, is not that inductive inference is invalid or that we should cease to reason from experience. As an agent I am perfectly satisfied; it is as a philosopher that the difficulty presses.9 Custom does the work that reason cannot do — but custom is not reason, and we ought at least to be sensible of our ignorance even if we cannot augment our knowledge.
all reasonings concerning cause and effect are founded on experience, and that all reasonings from experience are founded on the supposition, that the course of nature will continue uniformly the same.A Treatise of Human Nature, pp. 410–411
Adam with all his science, would never have been able to demonstrate, that the course of nature must continue uniformly the same, and that the future must be conformable to the past. What is possible can never be demonstrated to be false; and 'tis possible the course of nature may change, since we can conceive such a change.A Treatise of Human Nature, pp. 410–411
All probable arguments are built on the supposition, that there is this conformity betwixt the future and the past, and therefore can never prove it.A Treatise of Human Nature, pp. 410–411
When a man says, I have found, in all past instances, such sensible qualities conjoined with such secret powers: And when he says, Similar sensible qualities will always be conjoined with similar secret powers, he is not guilty of a tautology, nor are these propositions in any respect the same. You say that the one proposition is an inference from the other. But you must confess that the inference is not intuitive; neither is it demonstrative.An Enquiry Concerning Human Understanding, p. 13
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.An Enquiry Concerning Human Understanding, p. 13
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?An Enquiry Concerning Human Understanding, p. 13
When I see a billiard-ball moving towards another, my mind is immediately carried by habit to the usual effect, and anticipates my sight by conceiving the second ball in motion.A Treatise of Human Nature, pp. 410–411
the supposition, that the future resembles the past, is not founded on arguments of any kind, but is deriv'd entirely from habit, by which we are determin'd to expect for the future the same train of objects, to which we have been accustom'd.A Treatise of Human Nature, p. 92
My practice, you say, refutes my doubts. But you mistake the purport of my question. As an agent, I am quite satisfied in 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.An Enquiry Concerning Human Understanding, p. 13