David Hume, answered from the texts and cited to the page.
Custom and habit name the same underlying principle — the propensity of the mind, formed by repeated experience, to expect from the appearance of one object the appearance of another with which it has been constantly conjoined. When a man has seen flame followed by heat a thousand times, he finds himself determined to expect heat upon perceiving flame again; yet he has not, by all this experience, acquired any idea or knowledge of the secret power by which the one object produces the other, nor is it by any process of reasoning that he draws the inference.1
The understanding has no part in the operation; something else carries him forward, and that something is custom. The principle is this: wherever the repetition of any act or operation produces a propensity to renew the same act without being impelled by any reasoning, we call that propensity the effect of custom.2 I do not pretend, by employing that word, to have given the ultimate reason of such a propensity — I only point to a principle of human nature universally acknowledged and well known by its effects.3
Perhaps we cannot push our enquiries farther; perhaps we must rest contented with custom as the ultimate principle we can assign for all our conclusions from experience. What makes this remarkable is the contrast with reason. Reason draws the same conclusions from one circle as from all the circles in the universe; it cannot vary its conclusions with the number of instances.
But no man, having seen only one body move after being impelled by another, could infer that every other body will move after a like impulse.4 Custom explains precisely this variation: it is what allows a thousand instances to produce an inference that a single instance, in no respect different from them, could never produce. The billiard-ball makes the point vivid.
When I see one ball moving toward another, my mind is immediately carried by habit to the usual effect, anticipating the second ball in motion before my sight confirms it.5 There is nothing in these objects, abstractly considered and independent of experience, that leads me to form any such conclusion; the powers by which bodies operate are entirely unknown to us, and we perceive only their sensible qualities.
It is not reason that guides life here — it is custom, and custom alone determines the mind to suppose the future conformable to the past.6 The reach of the principle extends further still. Experience instructs me in the several conjunctions of objects for the past; habit determines me to expect the same for the future; and both conspiring together operate upon the imagination, enlivening certain ideas beyond others.7
Without this quality — seemingly trivial, little founded on reason — we could never assent to any argument, nor carry our view beyond those few objects immediately present to the senses. Memory, the senses, and the understanding are all, in this way, founded on the imagination, on the vivacity that habit confers upon our ideas.8 The evidence from animals confirms it.
Beasts certainly never perceive any real connexion among objects; they infer one from another by experience alone, and they can never by any arguments form a general conclusion about objects of which they have had no experience. It must therefore be by custom alone that experience operates upon them.9 And if custom suffices to explain the reasoning of beasts — where no suspicion of abstract ratiocination can arise — it is an invincible confirmation that the same principle governs human inference as well.
Reason, considered rightly, is nothing but a wonderful and unintelligible instinct in our souls, carrying us along a certain train of ideas; and that instinct, arising from past observation, is itself but one of the principles of nature — habit nothing but nature operating through repetition.10
Yet he has not, by all his experience, acquired any idea or knowledge of the secret power by which the one object produces the other; nor is it, by any process of reasoning, he is engaged to draw this inference. But still he finds himself determined to draw it.An Enquiry Concerning Human Understanding, p. 16
For wherever the repetition of any particular act or operation produces a propensity to renew the same act or operation, without being impelled by any reasoning or process of the understanding, we always say, that this propensity is the effect of Custom.An Enquiry Concerning Human Understanding, p. 16
By employing that word, we pretend not to have given the ultimate reason of such a propensity. We only point out a principle of human nature, which is universally acknowledged, and which is well known by its effects.An Enquiry Concerning Human Understanding, p. 16
Reason is incapable of any such variation. The conclusions which it draws from considering one circle are the same which it would form upon surveying all the circles in the universe. But no man, having seen only one body move after being impelled by another, could infer that every other body will move after a like impulse.An Enquiry Concerning Human Understanding, p. 16
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
'Tis not, therefore, reason, which is the guide of life, but custom. That alone determines the mind, in all instances, to suppose the future conformable to the past.A Treatise of Human Nature, pp. 410–411
Experience is a principle, which instructs me in the several conjunctions of objects for the past. Habit is another principle, which determines me to expect the same for the future; and both of them conspiring to operate upon the imagination, make me form certain ideas in a more intense and lively manner, than others.A Treatise of Human Nature, p. 173
The memory, senses, and understanding are, therefore, all of them founded on the imagination, or the vivacity of our ideas.A Treatise of Human Nature, p. 173
Beasts certainly never perceive any real connexion among objects. 'Tis therefore by experience they infer one from another. They can never by any arguments form a general conclusion, that those objects, of which they have had no experience, resemble those of which they have. 'Tis therefore by means of custom alone, that experience operates upon them.A Treatise of Human Nature, pp. 119–120
Reason is nothing but a wonderful and unintelligible instinct in our souls, which carries us along a certain train of ideas... This instinct, 'tis true, arises from past observation and experience... Nature may certainly produce whatever can arise from habit: Nay, habit is nothing but one of the principles of nature, and derives all its force from that origin.A Treatise of Human Nature, pp. 119–120