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An Enquiry Concerning Human Understanding. (1748)

Sect. IV. Sceptical Doubts concerning the Operations of the Understanding

David Hume.

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An Enquiry Concerning Human Understanding.

David Hume.

Sect. IV. Sceptical Doubts concerning the Operations of the Understanding

                               PART I.

  20. 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
hypothenuse 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 for ever retain their certainty and evidence.

  21. 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.

  22. All reasonings concerning matter of fact seem to be founded on
the realtion 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 connexion between the present
fact and that which is inferred from it. Were there nothing to bind
them together, the inference would be entirely precarious. The hearing
of an articulate voice and rational discourse in the dark assures us
of the presence of some person: Why? because these are the effects
of the human make and fabric, and closely connected with it. If we
anatomize all the other reasonings of this nature, we shall find
that they are founded on the relation of cause and effect, and that
this relation is either near or remote, direct or collateral. Heat and
light are collateral effects of fire, and the one effect may justly be
inferred from the other.

  23. 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.

  24. 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

  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.

  25. 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 connexion 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.

  26. 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

  27. 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 connexion 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.

                               PART II.

  28. 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.
This answer we must endeavour both to explain and to defend.

  29. 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 for ever 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
connexion 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 connexion 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.

  * The word, Power, is here used in a loose and popular sense. The
more accurate explication of it would give additional evidence to this
argument. See Sect. 7.

  30. This negative argument must certainly, in process of time,
become altogether convincing, if many penetrating and able
philosophers shall turn their enquiries this way and no one be ever
able to discover any connecting proposition or intermediate step,
which supports the understanding in this conclusion. But as the
question is yet new, every reader may not trust so far to his own
penetration, as to conclude, because an argument escapes his
enquiry, that therefore it does not really exist. For this reason it
may be requisite to venture upon a more difficult task; and
enumerating all the branches of human knowledge, endeavour to show
that none of them can afford such an argument.

  All reasonings may be divided into two kinds, namely,
demonstrative reasoning, or that concerning relations of ideas, and
moral reasoning, or that concerning matter of fact and existence. That
there are no demonstrative arguments in the case seems evident;
since it implies no contradiction that the course of nature may
change, and that an object, seemingly like those which we have
experienced, may be attended with different or contrary effects. May I
not clearly and distinctly conceive that a body, falling from the
clouds, and which, in all other respects, resembles snow, has yet
the taste of salt or feeling of fire? Is there any more intelligible
proposition than to affirm, that all the trees will flourish in
December and January, and decay in May and June? Now whatever is
intelligible, and can be distinctly conceived, implies no
contradiction, and can never be proved false by any demonstrative
argument or abstract reasoning a priori.

  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.

  31. 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 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
find, 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.

  32. Should it be said that, from a number of uniform experiments, we
infer a connexion 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 connexion
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. 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 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. No reading, no enquiry has yet been able to
remove my difficulty, or give me satisfaction in a matter of such
importance. Can I do better than propose the difficulty to the public,
even though, perhaps, I have small hopes of obtaining a solution? We
shall at least, by this means, be sensible of our ignorance, if we
do not augment our knowledge.

  33. 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

  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 pretence 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.

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