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An Enquiry Concerning Human Understanding. (1748)

Sect. VII. Of the Idea of necessary Connexion

David Hume.

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An Enquiry Concerning Human Understanding.

David Hume.

Sect. VII. Of the Idea of necessary Connexion

                             PART I.

  48 THE great advantage of the mathematical sciences above the
moral consists in this, that the ideas of the former, being
sensible, are always clear and determinate, the smallest distinction
between them is immediately perceptible, and the same terms are
still expressive of the same ideas, without ambiguity or variation. An
oval is never mistaken for a circle, nor an hyperbola for an ellipsis.
The isosceles and scalenum are distinguished by boundaries more
exact than vice and virtue, right and wrong. If any term be defined in
geometry, the mind readily, of itself, substitutes, on all
occasions, the definition for the term defined: Or even when no
definition is employed, the object itself may be presented to the
senses, and by that means be steadily and clearly apprehended. But the
finer sentiments of the mind, the operations of the understanding, the
various agitations of the passions, though really in themselves
distinct, easily escape us, when surveyed by reflection; nor is it
in our power to recal the original object, as often as we have
occasion to contemplate it. Ambiguity, by this means, is gradually
introduced into our reasonings: Similar objects are readily taken to
be the same: And the conclusion becomes at last very wide of the

  One may safely, however, affirm, that, if we consider these sciences
in a proper light, their advantages and disadvantages nearly
compensate each other, and reduce both of them to a state of equality.
If the mind, with greater facility, retains the ideas of geometry
clear and determinate, it must carry on a much longer and more
intricate chain of reasoning, and compare ideas much wider of each
other, in order to reach the abstruser truths of that science. And
if moral ideas are apt, without extreme care, to fall into obscurity
and confusion, the inferences are always much shorter in these
disquisitions, and the intermediate steps, which lead to the
conclusion, much fewer than in the sciences which treat of quantity
and number. In reality, there is scarcely a proposition in Euclid so
simple, as not to consist of more parts, than are to be found in any
moral reasoning which runs not into chimera and conceit. Where we
trace the principles of the human mind through a few steps, we may
be very well satisfied with our progress; considering how soon
nature throws a bar to all our enquiries concerning causes, and
reduces us to an acknowledgment of our ignorance. The chief
obstacle, therefore, to our improvement in the moral or metaphysical
sciences is the obscurity of the ideas, and ambiguity of the terms.
The principal difficulty in the mathematics is the length of
inferences and compass of thought, requisite to the forming of any
conclusion. And, perhaps, our progress in natural philosophy is
chiefly retarded by the want of proper experiments and phaenomena,
which are often discovered by chance, and cannot always be found, when
requisite, even by the most diligent and prudent enquiry. As moral
philosophy seems hitherto to have received less improvement than
either geometry or physics, we may conclude, that, if there be any
difference in this respect among these sciences, the difficulties,
which obstruct the progress of the former, require superior care and
capacity to be surmounted.

  49. There are no ideas, which occur in metaphysics, more obscure and
uncertain, than those of power, force, energy or necessary
connexion, of which it is every moment necessary for us to treat in
all our disquisitions. We shall, therefore, endeavour, in this
section, to fix, if possible, the precise meaning of these terms,
and thereby remove some part of that obscurity, which is so much
complained of in this species of philosophy.

  It seems a proposition, which will not admit of much dispute, that
all our ideas are nothing but copies of our impressions, or, in
other words, that it is impossible for us to think of anything,
which we have not antecedently felt, either by our external or
internal senses. I have endeavoured* to explain and prove this
proposition, and have expressed my hopes, that, by a proper
application of it, men may reach a greater clearness and precision
in philosophical reasonings, than what they have hitherto been able to
attain. Complex ideas may, perhaps, be well known by definition, which
is nothing but an enumeration of those parts or simple ideas, that
compose them. But when we have pushed up definitions to the most
simple ideas, and find still some ambiguity and obscurity; what
resource are we then possessed of? By what invention can we throw
light upon these ideas, and render them altogether precise and
determinate to our intellectual view? Produce the impressions or
original sentiments, from which the ideas are copied. These
impressions are all strong and sensible. They admit not of
ambiguity. They are not only placed in a full light themselves, but
may throw light on their correspondent ideas, which lie in
obscurity. And by this means, we may, perhaps, attain a new microscope
or species of optics, by which, in the moral sciences, the most
minute, and most simple ideas may be so enlarged as to fall readily
under our apprehension, and be equally known with the grossest and
most sensible ideas, that can be the object of our enquiry.

  * Section II.

  50. To be fully acquainted, therefore, with the idea of power or
necessary connexion, let us examine its impression; and in order to
find the impression with greater certainty, let us search for it in
all the sources, from which it may possibly be derived.

  When we look about us towards external objects, and consider the
operation of causes, we are never able, in a single instance, to
discover any power or necessary connexion; any quality, which binds
the effect to the cause, and renders the one an infallible consequence
of the other. We only find, that the one does actually, in fact,
follow the other. The impulse of one billiard-ball is attended with
motion in the second. This is the whole that appears to the outward
senses. The mind feels no sentiment or inward impression from this
succession of objects: Consequently, there is not, in any single,
particular instance of cause and effect, anything which can suggest
the idea of power or necessary connexion.

  From the first appearance of an object, we never can conjecture what
effect will result from it. But were the power or energy of any
cause discoverable by the mind, we could foresee the effect, even
without experience; and might, at first, pronounce with certainty
concerning it, by mere dint of thought and reasoning.

  In reality, there is no part of matter, that does ever, by its
sensible qualities, discover any power or energy, or give us ground to
imagine, that it could produce any thing, or be followed by any
other object, which we could denominate its effect. Solidity,
extension, motion; these qualities are all complete in themselves, and
never point out any other event which may result from them. The scenes
of the universe are continually shifting, and one object follows
another in an uninterrupted succession; but the power of force,
which actuates the whole machine, is entirely concealed from us, and
never discovers itself in any of the sensible qualities of body. We
know, that, in fact, heat is a constant attendant of flame; but what
is the connexion between them, we have no room so much as to
conjecture or imagine. It is impossible, therefore, that the idea of
power can be derived from the contemplation of bodies, in single
instances of their operation; because no bodies ever discover any
power, which can be the original of this idea.*

  * Mr. Locke, in his chapter of power, says that, finding from
experience, that there are several new productions in matter, and
concluding that there must somewhere be a power capable of producing
them, we arrive at last by this reasoning at the idea of power. But no
reasoning can ever give us a new, original, simple idea; as this
philosopher himself confesses. This, therefore, can never be the
origin of that idea.

  51. Since, therefore, external objects as they appear to the senses,
give us no idea of power or necessary connexion, by their operation in
particular instances, let us see, whether this idea be derived from
reflection on the operations of our own minds, and be copied from
any internal impression. It may be said, that we are every moment
conscious of internal power; while we feel, that, by the simple
command of our will, we can move the organs of our body, or direct the
faculties of our mind. An act of volition produces motion in our
limbs, or raises a new idea in our imagination. This influence of
the will we know by consciousness. Hence we acquire the idea of
power or energy; and are certain, that we ourselves and all other
intelligent beings are possessed of power. This idea, then, is an idea
of reflection, since it arises from reflecting on the operations of
our own mind, and on the command which is exercised by will, both over
the organs of the body and faculties of the soul.

  52. We shall proceed to examine this pretension; and first with
regard to the influence of volition over the organs of the body.
This influence, we may observe, is a fact, which, like all other
natural events, can be known only be experience, and can never be
foreseen from any apparent energy or power in the cause, which
connects it with the effect, and renders the one an infallible
consequence of the other. The motion of our body follows upon the
command of our will. Of this we are every moment conscious. But the
means, by which this is effected; the energy, by which the will
performs so extraordinary an operation; of this we are so far from
being immediately conscious, that it must for ever escape our most
diligent enquiry.

  For first; is there any principle in all nature more mysterious than
the union of soul with body; by which a supposed spiritual substance
acquires such an influence over a material one, that the most
refined thought is able to actuate the grossest matter? Were we
empowered, by a secret wish, to remove mountains, or control the
planets in their orbit; this extensive authority would not be more
extraordinary, nor more beyond our comprehension. But if by
consciousness we perceived any power or energy in the will, we must
know this power; we must know its connexion with the effect; we must
know the secret union of soul and body, and the nature of both these
substances; by which the one is able to operate, in so many instances,
upon the other.

  Secondly, We are not able to move all the organs of the body with
a like authority; though we cannot assign any reason besides
experience, for so remarkable a difference between one and the
other. Why has the will an influence over the tongue and fingers,
not over the heart or liver? This question would never embarrass us,
were we conscious of a power in the former case, not in the latter. We
should then perceive, independent of experience, why the authority
of will over the organs of the body is circumscribed within such
particular limits. Being in that case fully acquainted with the
power or force, by which it operates, we should also know, why its
influence reaches precisely to such boundaries, and no farther.

  A man, suddenly struck with palsy in the leg or arm, or who had
newly lost those members, frequently endeavours, at first to move
them, and employ them in their usual offices. Here he is as much
conscious of power to command such limbs, as a man in perfect health
is conscious of power to actuate any member which remains in its
natural state and condition. But consciousness never deceives.
Consequently, neither in the one case nor in the other, are we ever
conscious of any power. We learn the influence of our will from
experience alone. And experience only teaches us, how one event
constantly follows another; without instructing us in the secret
connexion, which binds them together, and renders them inseparable.

  Thirdly, We learn from anatomy, that the immediate object of power
in voluntary motion, is not the member itself which is moved, but
certain muscles, and nerves, and animal spirits, and, perhaps,
something still more minute and more unknown, through which the motion
is successively propagated, ere it reach the member itself whose
motion is the immediate object of volition. Can there be a more
certain proof, that the power, by which this whole operation is
performed, so far from being directly and fully known by an inward
sentiment or consciousness, is, to the last degree, mysterious and
unintelligible? Here the mind wills a certain event: Immediately
another event, unknown to ourselves, and totally different from the
one intended, is produced: This event produces another, equally
unknown: Till at last, through a long succession, the desired event is
produced. But if the original power were felt, it must be known:
Were it known, its effect also must be known; since all power is
relative to its effect. And vice versa, if the effect be not known,
the power cannot be known nor felt. How indeed can we be conscious
of a power to move our limbs, when we have no such power; but only
that to move certain animal spirits, which, though they produce at
last the motion of our limbs, yet operate in such a manner as is
wholly beyond our comprehension?

  We may, therefore, conclude from the whole, I hope, without any
temerity, though with assurance; that our idea of power is not
copied from any sentiment or consciousness of power within
ourselves, when we give rise to animal motion, or apply our limbs to
their proper use and office. That their motion follows the command
of the will is a matter of common experience, like other natural
events: But the power or energy by which this is effected, like that
in other natural events, is unknown and inconceivable.*

  * It may be pretended, that the resistance which we meet with in
bodies, obliging us frequently to exert our force, and call up all our
power, this gives us the idea of force and power. It is this nisus, or
strong endeavour, of which we are conscious, that is the original
impression from which this idea is copied. But, first, we attribute
power to a vast number of objects, where we never can suppose this
resistance of exertion of force to take place; to the Supreme Being,
who never meets with any resistance; to the mind in its command over
its ideas and limbs, in common thinking and motion, where the effect
follows immediately upon the will, without any exertion or summoning
up of force; to inanimate matter, which is not capable of this
sentiment. Secondly, This sentiment of an endeavour to overcome
resistance has no known connexion with any event: What follows it,
we know by experience; but could not know it a priori. It must,
however, be confessed, that the animal nisus, which we experience,
though it can afford no accurate precise idea of power, enters very
much into that vulgar, inaccurate idea, which is formed of it.

  53. Shall we then assert, that we are conscious of a power or energy
in our own minds, when, by an act or command of our will, we raise
up a new idea, fix the mind to the contemplation of it, turn it on all
sides, and at last dismiss it for some other idea, when we think
that we have surveyed it with sufficient accuracy? I believe the
same arguments will prove, that even this command of the will gives us
no real idea of force or energy.

  First, It must be allowed, that, when we know a power, we know
that very circumstance in the cause, by which it is enabled to produce
the effect: For these are supposed to be synonimous. We must,
therefore, know both the cause and effect, and the relation between
them. But do we pretend to be acquainted with the nature of the
human soul and the nature of an idea, or the aptitude of the one to
produce the other? This is a real creation; a production of
something out of nothing: Which implies a power so great, that it
may seem, at first sight, beyond the reach of any being, less than
infinite. At least it must be owned, that such a power is not felt,
nor known, nor even conceivable by the mind. We only feel the event,
namely, the existence of an idea, consequent to a command of the will:
But the manner, in which this operation is performed, the power by
which it is produced, is entirely beyond our comprehension.

  Secondly, The command of the mind over itself is limited, as well as
its command over the body; and these limits are not known by reason,
or any acquaintance with the nature of cause and effect, but only by
experience and observation, as in all other natural events and in
the operation of external objects. Our authority over our sentiments
and passions is much weaker than that over our ideas; and even the
latter authority is circumscribed within very narrow boundaries.
Will any one pretend to assign the ultimate reason of these
boundaries, or show why the power is deficient in one case, not in

  Thirdly, This self-command is very different at different times. A
man in health possesses more of it than one languishing with sickness.
We are more master of our thoughts in the morning than in the evening:
Fasting, than after a full meal. Can we give any reason for these
variations, except experience? Where then is the power, of which we
pretend to be conscious? Is there not here, either in a spiritual or
material substance, or both, some secret mechanism or structure of
parts, upon which the effect depends, and which, being entirely
unknown to us, renders the power or energy of the will equally unknown
and incomprehensible?

  Volition is surely an act of the mind, with which we are
sufficiently acquainted. Reflect upon it. Consider it on all sides. Do
you find anything in it like this creative power, by which it raises
from nothing a new idea, and with a kind of Fiat, imitates the
omnipotence of its Maker, if I may be allowed so to speak, who
called forth into existence all the various scenes of nature? So far
from being conscious of this energy in the will, it requires as
certain experience as that of which we are possessed, to convince us
that such extraordinary effects do ever result from a simple act of

  54. The generality of mankind never find any difficulty in
accounting for the more common and familiar operations of nature- such
as the descent of heavy bodies, the growth of plants, the generation
of animals, or the nourishment of bodies by food: But suppose that, in
all these cases, they perceive the very force or energy of the
cause, by which it is connected with its effect, and is for ever
infallible in its operation. They acquire, by long habit, such a
turn of mind, that, upon the appearance of the cause, they immediately
expect with assurance its usual attendant, and hardly conceive it
possible that any other event could result from it. It is only on
the discovery of extraordinary phaenomena, such as earthquakes,
pestilence, and prodigies of any kind, that they find themselves at
a loss to assign a proper cause, and to explain the manner in which
the effect is produced by it. It is usual for men, in such
difficulties, to have recourse to some invisible intelligent
principle* as the immediate cause of that event which surprises
them, and which, they think, cannot be accounted for from the common
powers of nature. But philosophers, who carry their scrutiny a
little farther, immediately perceive that, even in the most familiar
events, the energy of the cause is as unintelligible as in the most
unusual, and that we only learn by experience the frequent Conjunction
of objects, without being ever able to comprehend anything like
Connexion between them.

  * Theos apo mechanes (deus ex machina).

  55. Here, then, many philosophers think themselves obliged by reason
to have recourse, on all occasions, to the same principle, which the
vulgar never appeal to but in cases that appear miraculous and
supernatural. They acknowledge mind and intelligence to be, not only
the ultimate and original cause of all things, but the immediate and
sole cause of every event which appears in nature. They pretend that
those objects which are commonly denominated causes, are in reality
nothing but occasions; and that the true and direct principle of every
effect is not any power or force in nature, but a volition of the
Supreme Being, who wills that such particular objects should for
ever be conjoined with each other. Instead of saying that one
billiard-ball moves another by a force which it has derived from the
author of nature, it is the Deity himself, they say, who, by a
particular volition, moves the second ball, being determined to this
operation by the impulse of the first ball, in consequence of those
general laws which he has laid down to himself in the government of
the universe. But philosophers advancing still in their inquiries,
discover that, as we are totally ignorant of the power on which
depends the mutual operation of bodies, we are no less ignorant of
that power on which depends the operation of mind on body, or of
body on mind; nor are we able, either from our senses or
consciousness, to assign the ultimate principle in one case more
than in the other. The same ignorance, therefore, reduces them to
the same conclusion. They assert that the Deity is the immediate cause
of the union between soul and body; and that they are not the organs
of sense, which, being agitated by external objects, produce
sensations in the mind; but that it is a particular volition of our
omnipotent Maker, which excites such a sensation, in consequence of
such a motion in the organ. In like manner, it is not any energy in
the will that produces local motion in our members: It is God himself,
who is pleased to second our will, in itself impotent, and to
command that motion which we erroneously attribute to our own power
and efficacy. Nor do philosophers stop at this conclusion. They
sometimes extend the same inference to the mind itself, in its
internal operations. Our mental vision or conception of ideas is
nothing but a revelation made to us by our Maker. When we
voluntarily turn our thoughts to any object, and raise up its image in
the fancy, it is not the will which creates that idea: It is the
universal Creator, who discovers it to the mind, and renders it
present to us.

  56. Thus, according to these philosophers, every thing is full of
God. Not content with the principle, that nothing exists but by his
will, that nothing possesses any power but by his concession: They rob
nature, and all created beings, of every power, in order to render
their dependence on the Deity still more sensible and immediate.
They consider not that, by this theory, they diminish, instead of
magnifying, the grandeur of those attributes, which they affect so
much to celebrate. It argues surely more power in the Deity to
delegate a certain degree of power to inferior creatures than to
produce every thing by his own immediate volition. It argues more
wisdom to contrive at first the fabric of the world with such
perfect foresight that, of itself, and by its proper operation, it may
serve all the purposes of providence, than if the great Creator were
obliged every moment to adjust its parts, and animate by his breath
all the wheels of that stupendous machine.

  But if we would have a more philosophical confutation of this
theory, perhaps the two following reflections may suffice.

  57. First, it seems to me that this theory of the universal energy
and operation of the Supreme Being is too bold ever to carry
conviction with it to a man, sufficiently apprized of the weakness
of human reason, and the narrow limits to which it is confined in
all its operations. Though the chain of arguments which conduct to
it were ever so logical, there must arise a strong suspicion, if not
an absolute assurance, that it has carried us quite beyond the reach
of our faculties, when it leads to conclusions so extraordinary, and
so remote from common life and experience. We are got into fairy land,
long ere we have reached the last steps of our theory; and there we
have no reason to trust our common methods of argument, or to think
that our usual analogies and probabilities have any authority. Our
line is too short to fathom such immense abysses. And however we may
flatter ourselves that we are guided, in every step which we take,
by a kind of verisimilitude and experience, we may be assured that
this fancied experience has no authority when we thus apply it to
subjects that lie entirely out of the sphere of experience. But on
this we shall have occasion to touch afterwards.*

  * Section XII.

  Secondly, I cannot perceive any force in the arguments on which this
theory is founded. We are ignorant, it is true, of the manner in which
bodies operate on each other: Their force or energy is entirely
incomprehensible: But are we not equally ignorant of the manner or
force by which a mind, even the supreme mind, operates either on
itself or on body? Whence, I beseech you, do we acquire any idea of
it? We have no sentiment or consciousness of this power in
ourselves. We have no idea of the Supreme Being but what we learn from
reflection on our own faculties. Were our ignorance, therefore, a good
reason for rejecting anything, we should be led into that principle of
denying all energy in the Supreme Being as much as in the grossest
matter. We surely comprehend as little the operations of one as of the
other. Is it more difficult to conceive that motion may arise from
impulse than that it may arise from volition? All we know is our
profound ignorance in both cases.*

  * I need not examine at length the vis inertiae which is so much
talked of in the new philosophy, and which is ascribed to matter. We
find by experience, that a body at rest or in motion continues for
ever in its present state, till put from it by some new cause; and
that a body impelled takes as much motion from the impelling body as
it acquires itself. These are facts. When we call this a vis inertiae,
we only mark these facts, without pretending to have any idea of the
inert power; in the same manner as, when we talk of gravity, we mean
certain effects, without comprehending that active power. It was never
the meaning of Sir Isaac Newton to rob second causes of all force or
energy; though some of his followers have endeavoured to establish
that theory upon his authority. On the contrary, that great
philosopher had recourse to an etherial active fluid to explain his
universal attraction; though he was so cautious and modest as to
allow, that it was a mere hypothesis, not to be insisted on, without
more experiments. I must confess, that there is something in the
fate of opinions a little extraordinary. Descartes insinuated that
doctrine of the universal and sole efficacy of the Deity, without
insisting on it. Malebranche and other Cartesians made it the
foundation of all their philosophy. It had, however, no authority in
England. Locke, Clarke, and Cudworth, never so much as take notice
of it, but suppose all along, that matter has a real, though
subordinate and derived power. By what means has it become so
prevalent among our modern metaphysicians?

                             PART II.

  58. But to hasten to a conclusion of this argument, which is already
drawn out to too great a length: We have sought in vain for an idea of
power or necessary connexion in all the sources from which we could
suppose it to be derived. It appears that, in single instances of
the operation of bodies, we never can, by our utmost scrutiny,
discover anything but one event following another, without being
able to comprehend any force or power by which the cause operates,
or any connexion between it and its supposed effect. The same
difficulty occurs in contemplating the operations of mind on body-
where we observe the motion of the latter to follow upon the
volition of the former, but are not able to observe or conceive the
tie which binds together the motion and volition, or the energy by
which the mind produces this effect. The authority of the will over
its own faculties and ideas is not a whit more comprehensible: So
that, upon the whole, there appears not, throughout all nature, any
one instance of connexion which is conceivable by us. All events
seem entirely loose and separate. One event follows another; but we
never can observe any tie between them. They seem conjoined, but never
connected. And as we can have no idea of any thing which never
appeared to our outward sense or inward sentiment, the necessary
conclusion seems to be that we have no idea of connexion or power at
all, and that these words are absolutely without any meaning, when
employed either in philosophical reasonings or common life.

  59. But there still remains one method of avoiding this
conclusion, and one source which we have not yet examined. When any
natural object or event is presented, it is impossible for us, by
any sagacity or penetration, to discover, or even conjecture,
without experience, what event will result from it, or to carry our
foresight beyond that object which is immediately present to the
memory and senses. Even after one instance or experiment where we have
observed a particular event to follow upon another, we are not
entitled to form a general rule, or foretell what will happen in
like cases; it being justly esteemed an unpardonable temerity to judge
of the whole course of nature from one single experiment, however
accurate or certain. But when one particular species of event has
always, in all instances, been conjoined with another, we make no
any scruple of foretelling one upon the appearance of the other, and
of employing that reasoning, which can alone assure us of any matter
of fact or existence. We then call the one object, Cause; the other,
Effect. We suppose that there is some connexion between them; some
power in the one, by which it infallibly produces the other, and
operates with the greatest certainty and strongest necessity.

  It appears, then, that this idea of a necessary connexion among
events arises from a number of similar instances which occur of the
constant conjunction of these events; nor can that idea ever be
suggested by any one of these instances, surveyed in all possible
lights and positions. But there is nothing in a number of instances,
different from every single instance, which is supposed to be
exactly similar; except only, that after a repetition of similar
instances, the mind is carried by habit, upon the appearance of one
event, to expect its usual attendant, and to believe that it will
exist. This connexion, therefore, which we feel in the mind, this
customary transition of the imagination from one object to its usual
attendant, is the sentiment or impression from which we form the
idea of power or necessary connexion. Nothing farther is in the
case. Contemplate the subject on all sides; you will never find any
other origin of that idea. This is the sole difference between one
instance, from which we can never receive the idea of connexion, and a
number of similar instances, by which it is suggested. The first
time a man saw the communication of motion by impulse, as by the shock
of two billiard balls, he could not pronounce that the one event was
connected: but only that it was conjoined with the other. After he has
observed several instances of this nature, he then pronounces them
to be connected. What alteration has happened to give rise to this new
idea of connexion? Nothing but that he now feels these events to be
connected in his imagination, and can readily foretell the existence
of one from the appearance of the other. When we say, therefore,
that one object is connected with another, we mean only that they have
acquired a connexion in our thought, and give rise to this
inference, by which they become proofs of each other's existence: A
conclusion which is somewhat extraordinary, but which seems founded on
sufficient evidence. Nor will its evidence be weakened by any
general diffidence of the understanding, or sceptical suspicion
concerning every conclusion which is new and extraordinary. No
conclusions can be more agreeable to scepticism than such as make
discoveries concerning the weakness and narrow limits of human
reason and capacity.

  60. And what stronger instance can be produced of the surprising
ignorance and weakness of the understanding than the present? For
surely, if there be any relation among objects which it imports to
us to know perfectly, it is that of cause and effect. On this are
founded all our reasonings concerning matter of fact or existence.
By means of it alone we attain any assurance concerning objects
which are removed from the present testimony of our memory and senses.
The only immediate utility of all sciences, is to teach us, how to
control and regulate future events by their causes. Our thoughts and
enquiries are, therefore, every moment, employed about this
relation: Yet so imperfect are the ideas which we form concerning
it, that it is impossible to give any just definition of cause, except
what is drawn from something extraneous and foreign to it. Similar
objects are always conjoined with similar. Of this we have experience.
Suitably to this experience, therefore, we may define a cause to be an
object, followed by another, and where all the objects similar to
the first are followed by objects similar to the second. Or in other
words where, if the first object had not been, the second never had
existed. The appearance of a cause always conveys the mind, by a
customary transition, to the idea of the effect. Of this also we
have experience. We may, therefore, suitably to this experience,
form another definition of cause, and call it, an object followed by
another, and whose appearance always conveys the thought to that
other. But though both these definitions be drawn from circumstances
foreign to the cause, we cannot remedy this inconvenience, or attain
any more perfect definition, which may point out that circumstance
in the cause, which gives it a connexion with its effect. We have no
idea of this connexion, nor even any distinct notion what it is we
desire to know, when we endeavour at a conception of it. We say, for
instance, that the vibration of this string is the cause of this
particular sound. But what do we mean by that affirmation? We either
mean that this vibration is followed by this sound, and that all
similar vibrations have been followed by similar sounds: Or, that this
vibration is followed by this sound, and that upon the appearance of
one the mind anticipates the senses, and forms immediately an idea
of the other. We may consider the relation of cause and effect in
either of these two lights; but beyond these, we have no idea of it.*

  * According to these explications and definitions, the idea of power
is relative as much as that of cause; and both have a reference to
an effect, or some other event constantly conjoined with the former.
When we consider the unknown circumstance of an object, by which the
degree or quantity of its effect is fixed and determined, we call that
its power: And accordingly, it is allowed by all philosophers, that
the effect is the measure of the power. But if they had any idea of
power, as it is in itself, why could not they Measure it in itself?
The dispute whether the force of a body in motion be as its
velocity, or the square of its velocity; this dispute, I say, need not
be decided by comparing its effects in equal or unequal times; but
by a direct mensuration and comparison.

  As to the frequent use of the words, Force, Power, Energy, &c.,
which every where occur in common conversation, as well as in
philosophy; that is no proof, that we are acquainted, in any instance,
with the connecting principle between cause and effect, or can account
ultimately for the production of one thing to another. These words, as
commonly used, have very loose meanings annexed to them; and their
ideas are very uncertain and confused. No animal can put external
bodies in motion without the sentiment of a nisus or endeavour; and
every animal has a sentiment or feeling from the stroke or blow of
an external object that is in motion. These sensations, which are
merely animal, and from which we can a priori draw no inference, we
are apt to transfer to inanimate objects, and to suppose, that they
have some such feelings, whenever they transfer or receive motion.
With regard to energies, which are exerted, without our annexing to
them any idea of communicated motion, we consider only the constant
experienced conjunction of the events; and as we feel a customary
connexion between the ideas, we transfer that feeling to the
objects; as nothing is more usual than to apply to external bodies
every internal sensation, which they occasion.

  61. To recapitulate, therefore, the reasonings of this section:
Every idea is copied from some preceding impression or sentiment;
and where we cannot find any impression, we may be certain that
there is no idea. In all single instances of the operation of bodies
or minds, there is nothing that produces any impression, nor
consequently can suggest any idea of power or necessary connexion. But
when many uniform instances appear, and the same object is always
followed by the same event; we then begin to entertain the notion of
cause and connexion. We then feel a new sentiment or impression, to
wit, a customary connexion in the thought or imagination between one
object and its usual attendant; and this sentiment is the original
of that idea which we seek for. For as this idea arises from a
number of similar instances, and not from any single instance, it must
arise from that circumstance, in which the number of instances
differ from every individual instance. But this customary connexion or
transition of the imagination is the only circumstance in which they
differ. In every other particular they are alike. The first instance
which we saw of motion communicated by the shock of two billiard balls
(to return to this obvious illustration) is exactly similar to any
instance that may, at present, occur to us; except only, that we could
not, at first, infer one event from the other; which we are enabled to
do at present, after so long a course of uniform experience. I know
not whether the reader will readily apprehend this reasoning. I am
afraid that, should I multiply words about it, or throw it into a
greater variety of lights, it would only become more obscure and
intricate. In all abstract reasonings there is one point of view
which, if we can happily hit, we shall go farther towards illustrating
the subject than by all the eloquence and copious expression in the
world. This point of view we should endeavour to reach, and reserve
the flowers of rhetoric for subjects which are more adapted to them.

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