Hume decoded

David Hume: An Enquiry Concerning Human Understanding, edited


Everyone will readily allow that there is a considerable difference between the perceptions of the mind, when a man feels the pain of excessive heat, or the pleasure of moderate warmth, and when he afterwards recalls to his memory this sensation, or anticipates it by his imagination. These faculties may mimic or copy the perceptions of the senses; but they never can entirely reach the force and vivacity of the original sentiment. The utmost we say of them, even when they operate with greatest vigour, is that they represent their object in so lively a manner that we could almost say we feel or see it. But, except the mind be disordered by disease or madness, they never can arrive at such a pitch of vivacity as to render these perceptions altogether undistinguishable. All the colours of poetry, however splendid, can never paint natural objects in such a manner as to make the description be taken for a real landscape. The most lively thought is still inferior to the dullest sensation.

We may observe a like distinction to run through all the other perceptions of the mind. A man in a fit of anger is actuated in a very different manner from one who only thinks of that emotion. If you tell me that any person is in love, I easily understand your meaning and form a just conception of his situation, but never can mistake that conception for the real disorders and agitations of the passion. When we reflect on our past sentiments and affections, our thought is a faithful mirror, and copies its objects truly; but the colours which it employs are faint and dull, in comparison of those in which our original perceptions were clothed.

Here therefore we may divide all the perceptions of the mind into two classes or species, which are distinguished by their different degrees of force and vivacity. The less forcible and lively are commonly denominated ‘thoughts’ or ‘ideas’. The other species want a name in our language, and in most others; I suppose, because it was not requisite for any, but philosophical purposes, to rank them under a general term or appellation. Let us, therefore, use a little freedom, and call them ‘impressions’; employing that word in a sense somewhat different from the usual. By the term impression, then, I mean all our more lively perceptions, when we hear, or see, or feel, or love, or hate, or desire, or will.

Nothing, at first view, may seem more unbounded than the thought of man, which not only escapes all human power and authority, but is not even restrained within the limits of nature and reality. To form monsters, and join incongruous shapes and appearances costs the imagination no more trouble than to conceive the most natural and familiar objects. And while the body is confined to one planet, along which it creeps with pain and difficulty; the thought can in an instant transport us into the most distant regions of the universe; or even beyond the universe, into the unbounded chaos, where nature is supposed to lie in total confusion. What never was seen, or heard of, may yet be conceived; nor is anything beyond the power of thought, except what implies an absolute contradiction.[1]

But though our thought seems to possess this unbounded liberty, we shall find, upon a nearer examination, that it is really confined within very narrow limits, and that all this creative power of the mind amounts to no more than the faculty of compounding, transposing, augmenting, or diminishing the materials afforded us by the senses and experience.[2] When we think of a golden mountain, we only join two consistent ideas, gold, and mountain, with which we were formerly acquainted. In short, all the materials of thinking are derived either from our outward or inward sentiment: the mixture and composition of these belongs alone to the mind and will. Or, to express myself in philosophical language, all our ideas or more feeble perceptions are copies of our impressions or more lively ones.

To prove this, the two following arguments will, I hope, be sufficient. First, when we analyze our thoughts or ideas, however compounded or sublime, we always find that they resolve themselves into such simple ideas as were copied from a precedent feeling or sentiment. Even those ideas, which, at first view, seem the most wide of this origin, are found, upon a nearer scrutiny, to be derived from it. The idea of God, as meaning an infinitely intelligent, wise, and good Being, arises from reflecting on the operations of our own mind and augmenting, without limit, those qualities of goodness and wisdom. We may prosecute this enquiry to what length we please; where we shall always find, that every idea which we examine is copied from a similar impression. Those who would assert that this position is not universally true nor without exception, have only one, and that an easy method of refuting it; by producing that idea, which, in their opinion, is not derived from this source. It will then be incumbent on us, if we would maintain our doctrine, to produce the impression, or lively perception, which corresponds to it.

Secondly. If it happen, from a defect of the organ, that a man is not susceptible of any species of sensation, we always find that he is as little susceptible of the correspondent ideas. A blind man can form no notion of colours; a deaf man of sounds. Restore either of them that sense in which he is deficient; by opening this new inlet for his sensations, you also open an inlet for the ideas; and he finds no difficulty in conceiving these objects. The case is the same, if the object, proper for exciting any sensation, has never been applied to the organ.

There is, however, one contradictory phenomenon. I believe it will readily be allowed that the several distinct ideas of colour, which enter by the eye, or those of sound, which are conveyed by the ear, are really different from each other; though, at the same time, resembling. Now if this be true of different colours, it must be no less so of the different shades of the same colour; and each shade produces a distinct idea, independent of the rest. For if this should be denied, it is possible, by the continual gradation of shades, to run a colour insensibly into what is most remote from it; and if you will not allow any of the means to be different, you cannot, without absurdity, deny the extremes to be the same. Suppose, therefore, a person to have enjoyed his sight for thirty years, and to have become perfectly acquainted with colours of all kinds except one particular shade of blue, for instance, which it never has been his fortune to meet with. Let all the different shades of that colour, except that single one, be placed before him, descending gradually from the deepest to the lightest; it is plain that he will perceive a blank, where that shade is wanting, and will be sensible that there is a greater distance in that place between the contiguous colours than in any other. Now I ask, whether it be possible for him, from his own imagination, to supply this deficiency, and raise up to himself the idea of that particular shade, though it had never been conveyed to him by his senses? I believe there are few but will be of opinion that he can: and this may serve as a proof that the simple ideas are not always, in every instance, derived from the correspondent impressions; though this instance is so singular, that it is scarcely worth our observing, and does not merit that for it alone we should alter our general maxim.[3]

Here, therefore, is a proposition, which not only seems, in itself, simple and intelligible; but, if a proper use were made of it, might render every dispute equally intelligible, and banish all that jargon, which has so long taken possession of metaphysical reasonings, and drawn disgrace upon them.[4] All ideas, especially abstract ones, are naturally faint and obscure: the mind has but a slender hold of them: they are apt to be confounded with other resembling ideas; and when we have often employed any term, though without a distinct meaning, we are apt to imagine it has a determinate idea annexed to it. On the contrary, all impressions, that is, all sensations, either outward or inward, are strong and vivid: the limits between them are more exactly determined: nor is it easy to fall into any error or mistake with regard to them. When we entertain, therefore, any suspicion that a philosophical term is employed without any meaning or idea (as is but too frequent), we need but enquire, from what impression is that supposed idea derived? And if it be impossible to assign any, this will serve to confirm our suspicion. By bringing ideas into so clear a light we may reasonably hope to remove all dispute, which may arise, concerning their nature and reality.


It is evident that there is a principle of connection between the different thoughts or ideas of the mind, and that, in their appearance to the memory or imagination, they introduce each other with a certain degree of method and regularity. In our more serious thinking or discourse this is so observable that any particular thought, which breaks in upon the regular tract or chain of ideas, is immediately remarked and rejected. And even in our wildest and most wandering reveries, nay in our very dreams, we shall find, if we reflect, that the imagination ran not altogether at adventures, but that there was still a connexion upheld among the different ideas, which succeeded each other. Were the loosest and freest conversation to be transcribed, there would immediately be observed something which connected it in all its transitions. Or where this is wanting, the person who broke the thread of discourse might still inform you, that there had secretly revolved in his mind a succession of thought, which had gradually led him from the subject of conversation. Among different languages, even where we cannot suspect the least connection or communication, it is found, that the words, expressive of ideas, the most compounded, do yet nearly correspond to each other: a certain proof that the simple ideas, comprehended in the compound ones, were bound together by some universal principle, which had an equal influence on all mankind.

Though it be too obvious to escape observation, that different ideas are connected together; I do not find that any philosopher has attempted to enumerate or class all the principles of association; a subject, however, that seems worthy of curiosity. To me, there appear to be only three principles of connection among ideas, namely, Resemblance, Contiguity in time or place, and Cause or Effect.

That these principles serve to connect ideas will not, I believe, be much doubted. A picture naturally leads our thoughts to the original: the mention of one apartment in a building naturally introduces an enquiry or discourse concerning the others: and if we think of a wound, we can scarcely forbear reflecting on the pain which follows it. But that this enumeration is complete, and that there are no other principles of association except these, may be difficult to prove to the satisfaction of the reader, or even to a man’s own satisfaction. All we can do, in such cases, is to run over several instances, and examine carefully the principle which binds the different thoughts to each other, never stopping till we render the principle as general as possible. The more instances we examine, and the more care we employ, the more assurance shall we acquire, that the enumeration, which we form from the whole, is complete and entire.



All the objects of human reason or enquiry may naturally be divided into two kinds, to wit, Relations of Ideas, and Matters of Fact. Of the first kind are the sciences of geometry, algebra, and arithmetic; and in short, every affirmation which is either intuitively or demonstratively certain. That the square of the hypotenuse is equal to the square of the two sides is a proposition which expresses a relation between these figures. That three times five is equal to the half of thirty, expresses a relation between these numbers. Propositions of this kind are discoverable by the mere operation of thought, without dependence on what is anywhere existent in the universe. Though there never were a circle or triangle in nature, the truths demonstrated by Euclid would forever retain their certainty and evidence.

Matters of fact, which are the second objects of human reason, are not ascertained in the same manner; nor is our evidence of their truth, however great, of a like nature with the foregoing. The contrary of every matter of fact is still possible; because it can never imply a contradiction, and is conceived by the mind with the same facility and distinctness, as if ever so conformable to reality. That the sun will not rise tomorrow is no less intelligible a proposition and implies no more contradiction than the affirmation that it will rise. We should in vain, therefore, attempt to demonstrate its falsehood. Were it demonstratively false, it would imply a contradiction, and could never be distinctly conceived by the mind.

It may, therefore, be a subject worthy of curiosity, to enquire what is the nature of that evidence which assures us of any real existence and matter of fact, beyond the present testimony of our senses, or the records of our memory. This part of philosophy, it is observable, has been little cultivated, either by the ancients or moderns; and therefore our doubts and errors, in the prosecution of so important an enquiry, may be the more excusable; while we march through such difficult paths without any guide or direction. They may even prove useful, by exciting curiosity, and destroying that implicit faith and security, which is the bane of all reasoning and free enquiry. The discovery of defects in the common philosophy, if any such there be, will not, I presume, be a discouragement, but rather an incitement, as is usual, to attempt something more full and satisfactory than has yet been proposed to the public.

All reasonings concerning matter of fact seem to be founded on the relation of Cause and Effect. By means of that relation alone we can go beyond the evidence of our memory and senses. If you were to ask a man, why he believes any matter of fact, which is absent; for instance, that his friend is in the country, or in France; he would give you a reason; and this reason would be some other fact, as a letter received from him, or the knowledge of his former resolutions and promises. A man finding a watch or any other machine in a desert island, would conclude that there had once been men in that island. All our reasonings concerning fact are of the same nature. And here it is constantly supposed that there is a connection between the present fact and that which is inferred from it. Were there nothing to bind them together, the inference would be entirely precarious.

If we would satisfy ourselves, therefore, concerning the nature of that evidence, which assures us of matters of fact, we must enquire how we arrive at the knowledge of cause and effect.

I shall venture to affirm, as a general proposition, which admits of no exception, that the knowledge of this relation is not, in any instance, attained by reasonings a priori; but arises entirely from experience, when we find that any particular objects are constantly conjoined with each other. Let an object be presented to a man of ever so strong natural reason and abilities; if that object be entirely new to him, he will not be able, by the most accurate examination of its sensible qualities, to discover any of its causes or effects. Adam, though his rational faculties be supposed, at the very first, entirely perfect, could not have inferred from the fluidity and transparency of water that it would suffocate him, or from the light and warmth of fire that it would consume him. No object ever discovers, by the qualities which appear to the senses, either the causes which produced it, or the effects which will arise from it; nor can our reason, unassisted by experience, ever draw any inference concerning real existence and matter of fact.

This proposition, that causes and effects are discoverable, not by reason but by experience, will readily be admitted with regard to such objects, as we remember to have once been altogether unknown to us; since we must be conscious of the utter inability, which we then lay under, of foretelling what would arise from them. Present two smooth pieces of marble to a man who has no tincture of natural philosophy; he will never discover that they will adhere together in such a manner as to require great force to separate them in a direct line, while they make so small a resistance to a lateral pressure. Such events, as bear little analogy to the common course of nature, are also readily confessed to be known only by experience; nor does any man imagine that the explosion of gunpowder, or the attraction of a loadstone, could ever be discovered by arguments a priori. In like manner, when an effect is supposed to depend upon an intricate machinery or secret structure of parts, we make no difficulty in attributing all our knowledge of it to experience. Who will assert that he can give the ultimate reason, why milk or bread is proper nourishment for a man, not for a lion or a tiger?

But the same truth may not appear, at first sight, to have the same evidence with regard to events, which have become familiar to us from our first appearance in the world, which bear a close analogy to the whole course of nature, and which are supposed to depend on the simple qualities of objects, without any secret structure of parts. We are apt to imagine that we could discover these effects by the mere operation of our reason, without experience. We fancy, that were we brought on a sudden into this world, we could at first have inferred that one billiard-ball would communicate motion to another upon impulse; and that we needed not to have waited for the event, in order to pronounce with certainty concerning it. Such is the influence of custom, that, where it is strongest, it not only covers our natural ignorance, but even conceals itself, and seems not to take place, merely because it is found in the highest degree.

But 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. Motion in the second billiard-ball is a quite distinct event from motion in the first; nor is there anything in the one to suggest the smallest hint of the other. 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? And as the first imagination or invention of a particular effect, in all natural operations, is arbitrary, where we consult not experience; so must we also esteem the supposed tie or connection between the cause and effect, which binds them together, and renders it impossible that any other effect could result from the operation of that cause. When I see, for instance, a Billiard-ball moving in a straight line towards another; even suppose motion in the second ball should by accident be suggested to me, as the result of their contact or impulse; may I not conceive, that a hundred different events might as well follow from that cause? May not both these balls remain at absolute rest? May not the first ball return in a straight line, or leap off from the second in any line or direction? All these suppositions are consistent and conceivable. Why then should we give the preference to one, which is no more consistent or conceivable than the rest? All our reasonings a priori will never be able to show us any foundation for this preference.

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. It is confessed, that the utmost effort of human reason is to reduce the principles, productive of natural phenomena, to a greater simplicity, and to resolve the many particular effects into a few general causes, by means of reasonings from analogy, experience, and observation. But as to the causes of these general causes, we should in vain attempt their discovery; nor shall we ever be able to satisfy ourselves, by any particular explication of them. These ultimate springs and principles are totally shut up from human curiosity and enquiry. Elasticity, gravity, cohesion of parts, communication of motion by impulse; these are probably the ultimate causes and principles which we shall ever discover in nature; and we may esteem ourselves sufficiently happy, if, by accurate enquiry and reasoning, we can trace up the particular phenomena to, or near to, these general principles. The most perfect philosophy of the natural kind only staves off our ignorance a little longer: as perhaps the most perfect philosophy of the moral or metaphysical kind serves only to discover larger portions of it. Thus the observation of human blindness and weakness is the result of all philosophy, and meets us at every turn, in spite of our endeavours to elude or avoid it.

Nor is geometry, when taken into the assistance of natural philosophy, ever able to remedy this defect, or lead us into the knowledge of ultimate causes, by all that accuracy of reasoning for which it is so justly celebrated. Every part of mixed mathematics proceeds upon the supposition that certain laws are established by nature in her operations; and abstract reasonings are employed, either to assist experience in the discovery of these laws, or to determine their influence in particular instances, where it depends upon any precise degree of distance and quantity. Thus, it is a law of motion, discovered by experience, that the moment or force of any body in motion is in the compound ratio or proportion of its solid contents and its velocity; and consequently, that a small force may remove the greatest obstacle or raise the greatest weight, if, by any contrivance or machinery, we can increase the velocity of that force, so as to make it an overmatch for its antagonist. Geometry assists us in the application of this law, by giving us the just dimensions of all the parts and figures which can enter into any species of machine; but still the discovery of the law itself is owing merely to experience, and all the abstract reasonings in the world could never lead us one step towards the knowledge of it. When we reason a priori, and consider merely any object or cause, as it appears to the mind, independent of all observation, it never could suggest to us the notion of any distinct object, such as its effect; much less, show us the inseparable and inviolable connection between them. A man must be very sagacious who could discover by reasoning that crystal is the effect of heat, and ice of cold, without being previously acquainted with the operation of these qualities.


But we have not yet attained any tolerable satisfaction with regard to the question first proposed. Each solution still gives rise to a new question as difficult as the foregoing, and leads us on to farther enquiries. When it is asked, “What is the nature of all our reasonings concerning matter of fact?” The proper answer seems to be, that they are founded on the relation of cause and effect. When again it is asked, “What is the foundation of all our reasonings and conclusions concerning that relation?” It may be replied in one word, “Experience.” But if we still carry on our sifting humour, and ask, “What is the foundation of all conclusions from experience?” This implies a new question, which may be of more difficult solution and explication. Philosophers, that give themselves airs of superior wisdom and sufficiency, have a hard task when they encounter persons of inquisitive dispositions, who push them from every corner to which they retreat, and who are sure at last to bring them to some dangerous dilemma. The best expedient to prevent this confusion, is to be modest in our pretensions; and even to discover the difficulty ourselves before it is objected to us. By this means, we may make a kind of merit of our very ignorance.

I shall content myself, in this section, with an easy task, and shall pretend only to give a negative answer to the question here proposed. 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.[5] This answer we must endeavour both to explain and to defend.

It must certainly be allowed, that nature has kept us at a great distance from all her secrets, and has afforded us only the knowledge of a few superficial qualities of objects; while she conceals from us those powers and principles on which the influence of those objects entirely depends. Our senses inform us of the colour, weight, and consistence of bread; but neither sense nor reason can ever inform us of those qualities which fit it for the nourishment and support of a human body. Sight or feeling conveys an idea of the actual motion of bodies; but as to that wonderful force or power, which would carry on a moving body forever in a continued change of place, and which bodies never lose but by communicating it to others; of this we cannot form the most distant conception. But notwithstanding this ignorance of natural powers and principles, we always presume, when we see like sensible qualities, that they have like secret powers, and expect that effects, similar to those which we have experienced, will follow from them. If a body of like colour and consistence with that bread, which we have formerly eat, be presented to us, we make no scruple of repeating the experiment, and foresee, with certainty, like nourishment and support. Now this is a process of the mind or thought, of which I would willingly know the foundation. It is allowed on all hands that there is no known connection between the sensible qualities and the secret powers; and consequently, that the mind is not led to form such a conclusion concerning their constant and regular conjunction, by anything which it knows of their nature. As to past experience, it can be allowed to give direct and certain information of those precise objects only, and that precise period of time, which fell under its cognizance: but why this experience should be extended to future times, and to other objects, which for aught we know, may be only in appearance similar; this is the main question on which I would insist. The bread, which I formerly eat, 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 if you insist that the inference is made by a chain of reasoning, I desire you to produce that reasoning. The connection between these propositions is not intuitive. There is required a medium, which may enable the mind to draw such an inference, if indeed it be drawn by reasoning and argument. What that medium is, I must confess, passes my comprehension; and it is incumbent on those to produce it, who assert that it really exists, and is the origin of all our conclusions concerning matter of fact.

If we be, therefore, engaged by arguments to put trust in past experience, and make it the standard of our future judgement, these arguments must be probable only, or such as regard matter of fact and real existence, according to the division above mentioned. But that there is no argument of this kind must appear, if our explication of that species of reasoning be admitted as solid and satisfactory. We have said that all arguments concerning existence are founded on the relation of cause and effect; that our knowledge of that relation is derived entirely from experience; and that all our experimental conclusions proceed upon the supposition that the future will be conformable to the past. To endeavour, therefore, the proof of this last supposition by probable arguments, or arguments regarding existence, must be evidently going in a circle, and taking that for granted, which is the very point in question.[6]

In reality, all arguments from experience are founded on the similarity which we discover among natural objects, and by which we are induced to expect effects similar to those which we have found to follow from such objects. And though none but a fool or madman will ever pretend to dispute the authority of experience, or to reject that great guide of human life, it may surely be allowed a philosopher to have so much curiosity at least as to examine the principle of human nature, which gives this mighty authority to experience, and makes us draw advantage from that similarity which nature has placed among different objects. From causes which appear similar we expect similar effects. This is the sum of all our experimental conclusions. Now it seems evident that, if this conclusion were formed by reason, it would be as perfect at first, and upon one instance, as after ever so long a course of experience. But the case is far otherwise. Nothing is so like as eggs; yet no one, on account of this appearing similarity, expects the same taste and relish in all of them. It is only after a long course of uniform experiments in any kind, that we attain a firm reliance and security with regard to a particular event. Now where is that process of reasoning which, from one instance, draws a conclusion, so different from that which it infers from a hundred instances that are nowise different from that single one? This question I propose as much for the sake of information, as with an intention of raising difficulties. I cannot imagine any such reasoning. But I keep my mind still open to instruction, if any one will vouchsafe to bestow it on me.

Should it be said that, from a number of uniform experiments, we infer a connection between the sensible qualities and the secret powers; this, I must confess, seems the same difficulty, couched in different terms. The question still recurs, on what process of argument this inference is founded? Where is the medium, the interposing ideas, which join propositions so very wide of each other? It is confessed that the colour, consistence, and other sensible qualities of bread appear not, of themselves, to have any connection with the secret powers of nourishment and support. For otherwise we could infer these secret powers from the first appearance of these sensible qualities, without the aid of experience; contrary to the sentiment of all philosophers, and contrary to plain matter of fact. Here, then, is our natural state of ignorance with regard to the powers and influence of all objects. How is this remedied by experience? It only shows us a number of uniform effects, resulting from certain objects, and teaches us that those particular objects, at that particular time, were endowed with such powers and forces. When a new object, endowed with similar sensible qualities, is produced, we expect similar powers and forces, and look for a like effect. From a body of like colour and consistence with bread we expect like nourishment and support. But this surely is a step or progress of the mind, which wants to be explained. 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: Of what nature is it, then? To say it is experimental is begging the question.[7] 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.

I must confess that a man is guilty of unpardonable arrogance who concludes, because an argument has escaped his own investigation, that therefore it does not really exist. I must also confess that, though all the learned, for several ages, should have employed themselves in fruitless search upon any subject, it may still, perhaps, be rash to conclude positively that the subject must, therefore, pass all human comprehension. Even though we examine all the sources of our knowledge, and conclude them unfit for such a subject, there may still remain a suspicion, that the enumeration is not complete, or the examination not accurate. But with regard to the present subject, there are some considerations which seem to remove all this accusation of arrogance or suspicion of mistake.

It is certain that the most ignorant and stupid peasants—nay infants, nay even brute beasts—improve by experience, and learn the qualities of natural objects, by observing the effects which result from them. When a child has felt the sensation of pain from touching the flame of a candle, he will be careful not to put his hand near any candle; but will expect a similar effect from a cause which is similar in its sensible qualities and appearance. If you assert, therefore, that the understanding of the child is led into this conclusion by any process of argument or ratiocination, I may justly require you to produce that argument; nor have you any pretense to refuse so equitable a demand. You cannot say that the argument is abstruse, and may possibly escape your enquiry; since you confess that it is obvious to the capacity of a mere infant. If you hesitate, therefore, a moment, or if, after reflection, you produce any intricate or profound argument, you, in a manner, give up the question, and confess that it is not reasoning which engages us to suppose the past resembling the future, and to expect similar effects from causes which are, to appearance, similar. This is the proposition which I intended to enforce in the present section. If I be right, I pretend not to have made any mighty discovery. And if I be wrong, I must acknowledge myself to be indeed a very backward scholar; since I cannot now discover an argument which, it seems, was perfectly familiar to me long before I was out of my cradle.



David Hume: An Enquiry Concerning Human Understanding, decoded


It is well known that there is a big difference between sensations and ideas. Ideas are much less vivid and forceful. The idea of warmth is much different from actually feeling warm. Talking about love is much different from feeling actual love. While our thoughts are faithful and accurate, they are faint and dull compared to the original perceptions.

As a result, we can divide all perceptions into two kinds, depending on their force or vivacity. The less forceful perceptions are obviously ‘thoughts’, or ‘ideas’. The perceptions are a little harder to name because, I think, we don’t tend to talk about them all as a set, except in philosophy. I will call them ‘impressions’. By the term impression, I mean all the lively perceptions, like when we hear, see, feel, love, hate, desire, or will.

At first, it seems like our minds are free and limitless. We can imagine anything we desire. We can imagine monsters and other universes without any more effort than it takes to imagine the most familiar objects.

But while our thoughts seem free, they are actually quite confined. All we can do is compound, transpose, augment, or diminish the objects of our senses. When we think of a golden mountain, we join two ideas we already had: gold and mountain. Although the complete idea might be new, its components are not. All the objects of thought are given to us. Our minds can only remix them, and can never come up with something entirely fresh.

There are two proofs of this. The first is a challenge: if someone thinks it is possible to invent something new and not composed of impressions that preceded it, she need only produce the idea. If I cannot produce the impressions, she wins.

Second, we can see that people with some disabilities cannot even imagine the corresponding ideas. Blind people cannot imagine colours, and deaf people cannot imagine sounds.

I admit that there is one possible contradiction that I can imagine, and which may prove that there are some fresh ideas that do not come from senses. If a person had never seen one particular shade of blue, and if a rainbow of shades of blue were presented to her, but with that one shade missing, she will be able fill in the blank and imagine the shade she had never seen. Still, this is so small an exception as to be worthless, so I don’t think it changes my general rule: all ideas come originally from sensations.

Therefore, here is a proposition that seems simple yet is quite profound: when we consider a philosophical term or idea, we should enquire what impression it comes from. If taken seriously, this principle would abolish all the metaphysical nonsense people bandy about. Many abstract ideas are faint and obscure, and we become easily confused about them. We assume when we use some philosophical terms that they have a concrete meaning attached, but this is often not the case. Bringing misty ideas into the light will remove all dispute.


Some ideas are consistently followed by other ideas, and all ideas come from somewhere. Our thoughts are like links of a chain, and each thought is connected somehow to the one before it. This is even true in lighthearted conversations; when the thread of a conversation gets broken, the person who broke it can always say, “Oh, x reminded me of y, and that’s why I mentioned z.”

Strangely, no philosopher has ever listed all the principles of connection. There are only three ways to connect ideas: resemblance, contiguity in time or place, and cause and effect.

It is obvious that these relations exist; that they are the only relations is much harder to prove. All we can do is try several examples and see if there are others.


There are two objects of thought: relations of ideas and matters of fact.

Relations of ideas include geometry, algebra, and arithmetic, and all the other things that are intuitively or demonstratively certain. We can be absolutely certain that “three times five is half of thirty” is true. Thinking this statement expresses a certain relationship between the numbers. The truth of this kind of statement can be found through thinking alone and without looking to experience.

Matters of fact are different. They are not found to be true or false through mere thought, because it is logically possible that every matter of fact could be false. Whenever we need to determine whether a fact is true, we must always check our senses and our memories. For instance, the sentence “the sun will not rise tomorrow” makes perfect logical sense and is distinct. To prove it wrong, we must look outside tomorrow. This makes it quite different from logically false statements; we cannot get a clear idea of the statement “9+1=9”. In some way, the statement does not even make sense.

As far as I know, few philosophers have investigated what reasons we have to trust these matters of fact other than our senses and our memories. It seems to me that all our thinking about matters of fact is based on the relation of cause and effect. Cause and effect is what allows us to extrapolate from memory and sense. If you were to find a watch on a desert island, you would conclude that someone had been there before—that they had caused the watch to be there. All thoughts about facts are the same: heat and light are caused by fire, voices in the dark are caused by unseen people, and so on.

But how do we come to know about cause and effect?

Really, we don’t know much about it at all. We do not know cause and effect from a priori reasoning. When Adam and Eve were first shown water, they could never have known that it could suffocate them. If I gave you gunpowder, there is no way you could tell just from contemplating it that it explodes. Nor could you tell that bread nourishes you but does not nourish a lion.

This is hard to admit, but it seems to be true. We believe that we are born into the world able to see cause and effect, able to see that one billiard ball hitting another causes it to move. Yet it is not true. If we look very carefully and think very cautiously, we must admit that the mind never sees an effect in its cause. The effect is totally distinct and cannot be discovered without observation. The motion of the second billiard ball is distinct from the first, and there is nothing in the first that suggests the motion of the second. When I think about a billiard ball moving towards another, I can imagine many things happening: the second ball moves, the second ball does not move, both balls stop, or the second ball causes the first ball to rebound. All of these are consistent, clear, and conceivable. Why would I prefer one to the others?

Every effect is distinct from its cause and cannot be discovered in the cause, and any association with its cause must be entirely arbitrary. We simply cannot look at a thing or phenomenon and intuit through reason alone what effects it will have.

Scientists and philosophers have discovered a few general causes, like ultimate springs and principles. These are the things like elasticity, gravity, attraction, and so on. These are the general principles that govern nature. The trouble, though, is simple: explaining particular phenomena through general principles only postpones the question. How do we explain the general principle? We cannot explain what caused elasticity, gravity, or attraction.

Mathematics, physics, and geometry are of no use. They proceed from premises to conclusions. Geometry says that if x is the case, then y will happen, but it does not say that x is actually true, or even what x is. We need real experience to know whether something is true and what effects it might have.


We still haven’t reached the end of our questioning, though. Every time we answer a question, another one comes up. We ask, how do we reason about matters of fact? The answer is, all of our thoughts are all founded on the relation of cause and effect. Then we ask, how do we know about cause and effect? The answer: Experience. But how do we draw conclusions from experience? This is a very hard question.

Actually, this question is only hard if we try to answer it with a positive answer. It’s easy if we admit our limitations. I will give a negative answer: we cannot draw conclusions from experience.

Nature hides many things from us. For instance, we can see a ball moving, but we cannot see whether it can impart that movement onto another ball. That power is hidden. We eat brown bread, and it seems to nourish us. But how can we know that the next loaf will nourish us in the same way? We cannot. We imagine that we can, but there is nothing in the appearance of the bread that tells us it will. Of course, it always has, and it probably always will, but there is no way that I can see to actually prove it.

Even if you doubt me, you must admit that there is a difference between these two propositions:

  • I have always found this cause to have that effect, and
  • This cause will always have that effect

I know that everyone always infers the second proposition from the first, but I do not know what reasoning leads to the inference. There is no obvious connection between these two propositions.

There is an obvious but deceptive argument: Until now, the future has always resembled the past, so in the future, the future will resemble the past. This is arguing in a circle, though; we need to prove that the future will resemble the past. We can’t just assume it.

There is another, similar argument:

  • Most of our generalizations have been right
  • Therefore, most of our generalizations will be right

To make this argument, we need to make a generalization about our generalizations: that our generalizations are usually right. Clearly that, too, is assuming the very thing we want to prove.

Perhaps we observe the hidden capacities of things after a number of experiments. This seems to have the same difficulties, though. How do we infer what the capacities are? Why do we admit the nourishing capacity of bread after many instances, but not on the first? And how do we determine that bread will always possess the ability in the future? We need to see many instances of cause and effect before we judge the relation to be true. Who would believe that chickens lay eggs the first time they observed it? If the effect were judged by reason, one observation would be enough.

Of course, you may say that I am a bit of a hypocrite: I eat bread and trust that the sun will rise. I look both ways when I cross the street, because I don’t doubt that traffic retains the ability to kill me. As a person, I am quite satisfied that I should continue eating. Yet, as a philosopher, and as a skeptic, I want to know why.

Only the arrogant think that the things they can’t see don’t exist. It might even be that philosophers will look for ages before finally concluding I am wrong. Still, there are some considerations that might remove all doubt.

Infants and animals learn from experience. Even a baby knows learns from one accident that she cannot touch a flame. If a baby learns from reason or argument, I would like you to produce the reasons and the argument. If you can’t, well, the baby is smarter than you. And, of course, if you can, the baby is smarter than me.




[1] Kant will have much more to say on this. He calls it the principle of contradiction, and it’s the starting point for some deep philosophy.

[2] Descartes said this very thing in the Meditations.

[3] I cannot for the life of me imagine why Hume would admit this exception. It begs readers to find similar ones and to expand upon it. What about musical notes, for instance? I think the argument is even stronger. And how big a gap can the mind fill in? Expanding like this, bit by bit, we undermine Hume—perhaps fatally.

[4] Even philosophers realize that philosophers are often full of crap. Every once in a while, one comes along and tries to get us all talking sense. It never works and it never lasts.

[5] This is huge. He is saying that we know much less than we assume.

[6] This is the “problem of induction”. Induction is, roughly, science. We look at the world and draw generalizations about it. How can we do that when the world changes all the time, and we can’t see deep inside its mysterious workings? An example might help.

Alice and Biljana are in a bar. Biljana’s getting a divorce, and she turns to Alice, her lawyer friend, and says “Alice, I’m really stressed. I don’t know how to find a lawyer I can trust. How can you tell?”

Biljana’s eyes brighten. This is something she knows about. “Alice, it’s easy. You look for a lawyer whose shoes match their bag. It’s that simple! All the best lawyers colour coordinate.”

“Wait, wait! Really? That seems crazy.” Alice says. “How do I know I can trust your advice?”

Biljana replies, “Alice, just look at my shoes and bag! They match!”

Scientists are like lawyers. When they’re asked “How do you know that you can draw conclusions from evidence?”, they reply, “Easy—just look at the evidence! Most times when we draw conclusions from evidence, the conclusion’s correct!”

Like Biljana, they’ve missed the point: drawing a conclusion from evidence about drawing conclusions from evidence is arguing in a circle. The question is “why does induction work?” not “what evidence do we have that induction works?” To answer the second question, you use induction to show that induction works, assuming the very point you’re trying to prove.



[7] To philosophers “begging the question” means something entirely different. Question begging is assuming what you want to prove then pretending you proved it.