Philosophical Works, v. 1 · David Hume
OF SCEPTICISM WITH REGARD TO THE SENSES.
Thus the sceptic still continues to reason and believe, even though he asserts that he cannot defend his reason by reason; and by the same rule he must assent to the principle concerning the existence of body, though he cannot pretend, by any arguments of philosophy, to maintain its veracity. Nature has not left this to his choice, and has doubtless esteemed it an affair of too great importance, to be trusted to our uncertain reasonings and speculations. We may well ask, _What causes induce us to believe in the existence of body_? but 'tis in vain to ask, _Whether there be body or not_? That is a point, which we must take for granted in all our reasonings. The subject, then, of our present inquiry, is concerning the _causes_ which induce us to believe in the existence of body: and my reasonings on this head I shall begin with a distinction, which at first sight may seem superfluous, but which will contribute very much to the perfect understanding of what follows. We ought to examine apart those two questions, which are commonly confounded together, viz. Why we attribute a _continued_ existence to objects, even when they are not present to the senses; and why we suppose them to have an existence _distinct_ from the mind and perception? Under this last head I comprehend their situation as well as relations, their _external_ position as well as the _independence_ of their existence and operation. These two questions concerning the continued and distinct existence of body are intimately connected together. For if the objects of our senses continue to exist, even when they are not perceived, their existence is of course independent of and distinct from the perception; and _vice versa_, if their existence be independent of the perception, and distinct from it, they must continue to exist, even though they be not perceived. But though the decision of the one question decides the other; yet that we may the more easily discover the principles of human nature, from whence the decision arises, we shall carry along with us this distinction, and shall consider, whether it be the _senses, reason_, or the _imagination_, that produces the opinion of a _continued_ or of a _distinct_ existence. These are the only questions that are intelligible on the present subject. For as to the notion of external existence, when taken for something specifically different from our perceptions, we have already shown its absurdity.[1]
To begin with the _senses_,'tis evident these faculties are incapable of giving rise to the notion of the _continued_ existence of their objects, after they no longer appear to the senses. For that is a contradiction in terms, and supposes that the senses continue to operate, even after they have ceased all manner of operation. These faculties, therefore, if they have any influence in the present case, must produce the opinion of a distinct, not of a continued existence; and in order to that, must present their impressions either as images and representations, or as these very distinct and external existences. That our senses offer not their impressions as the images of something _distinct_, or _independent_, and _external_, is evident; because they convey to us nothing but a single perception, and never give us the least intimation of any thing beyond. A single perception can never produce the idea of a double existence, but by some inference either of the reason or imagination. When the mind looks farther than what immediately appears to it, its conclusions can never be put to the account of the senses; and it certainly looks farther, when from a single perception it infers a double existence, and supposes the relations of resemblance and causation betwixt them.
If our senses, therefore, suggest any idea of distinct existences, they must convey the impressions as those very existences, by a kind of fallacy and illusion. Upon this head we may observe, that all sensations are felt by the mind, such as they really are, and that, when we doubt whether they present themselves as distinct objects, or as mere impressions, the difficulty is not concerning their nature, but concerning their relations and situation. Now, if the senses presented our impressions as external to, and independent of ourselves, both the objects and ourselves must be obvious to our senses, otherwise they could not be compared by these faculties. The difficulty then, is, how far we are _ourselves_ the objects of our senses. 'Tis certain there is no question in philosophy more abstruse than that concerning identity, and the nature of the uniting principle, which constitutes a person. So far from being able by our senses merely to determine this question, we must have recourse to the most profound metaphysics to give a satisfactory answer to it; and in common life 'tis evident these ideas of self and person are never very fixed nor determinate. 'Tis absurd therefore to imagine the senses can ever distinguish betwixt ourselves and external objects.
Add to this, that every impression, external and internal, passions, affections, sensations, pains and pleasures, are originally on the same footing; and that whatever other differences we may observe among them, they appear, all of them, in their true colours, as impressions or perceptions. And indeed, if we consider the matter aright, 'tis scarce possible it should be otherwise; nor is it conceivable that our senses should be more capable of deceiving us in the situation and relations, than in the nature of our impressions. For since all actions and sensations of the mind are known to us by consciousness, they must necessarily appear in every particular what they are, and be what they appear. Every thing that enters the mind, being in _reality_ as the perception, 'tis impossible any thing should to _feeling_ appear different. This were to suppose, that even where we are most intimately conscious, we might be mistaken. But not to lose time in examining, whether 'tis possible for our senses to deceive us, and represent our perceptions as distinct from ourselves, that is, as _external_ to and _independent_ of us; let us consider whether they really do so, and whether this error proceeds from an immediate sensation, or from some other causes.
To begin with the question concerning _external_ existence, it may perhaps be said, that setting aside the metaphysical question of the identity of a thinking substance, our own body evidently belongs to us; and as several impressions appear exterior to the body, we suppose them also exterior to ourselves. The paper, on which I write at present, is beyond my hand. The table is beyond the paper. The walls of the chamber beyond the table. And in casting my eye towards the window, I perceive a great extent of fields and buildings beyond my chamber. From all this it may be inferred, that no other faculty is required, beside the senses, to convince us of the external existence of body. But to prevent this inference, we need only weigh the three following considerations. _First_, that, properly speaking, 'tis not our body we perceive, when we regard our limbs and members, but certain impressions, which enter by the senses; so that the ascribing a real and corporeal existence to these impressions, or to their objects, is an act of the mind as difficult to explain as that which we examine at present. _Secondly_, sounds, and tastes, and smells, though commonly regarded by the mind as continued independent qualities, appear not to have any existence in extension, and consequently cannot appear to the senses as situated externally to the body. The reason why we ascribe a place to them, shall be considered afterwards.[2] _Thirdly_, even our sight informs us not of distance or outness (so to speak) immediately and without a certain reasoning and experience, as is acknowledged by the most rational philosophers.
As to the _independency_ of our perceptions on ourselves, this can never be an object of the senses; but any opinion we form concerning it, must be derived from experience and observation: and we shall see afterwards, that our conclusions from experience are far from being favourable to the doctrine of the independency of our perceptions. Meanwhile we may observe, that when we talk of real distinct existences, we have commonly more in our eye their independency than external situation in place, and think an object has a sufficient reality, when its being is uninterrupted, and independent of the incessant revolutions, which we are conscious of in ourselves. Thus to resume what I have said concerning the senses; they give us no notion of continued existence, because they cannot operate beyond the extent, in which they really operate. They as little produce the opinion of a distinct existence, because they neither can offer it to the mind as represented, nor as original. To offer it as represented, they must present both an object and an image. To make it appear as original, they must convey a falsehood; and this falsehood must lie in the relations and situation: in order to which, they must be able to compare the object with ourselves; and even in that case they do not, nor is it possible they should deceive us. We may therefore conclude with certainty, that the opinion of a continued and of a distinct existence never arises from the senses.
To confirm this, we may observe, that there are three different kinds of impressions conveyed by the senses. The first are those of the figure, bulk, motion, and solidity of bodies. The second, those of colours, tastes, smells, sounds, heat and cold. The third are the pains and pleasures that arise from the application of objects to our bodies, as by the cutting of our flesh with steel, and such like. Both philosophers and the vulgar suppose the first of these to have a distinct continued existence. The vulgar only regard the second as on the same footing. Both philosophers and the vulgar, again, esteem the third to be merely perceptions; and, consequently, interrupted and dependent beings. Now, 'tis evident, that, whatever may be our philosophical opinion, colour, sounds, heat and cold, as far as appears to the senses, exist after the same manner with motion and solidity; and that the difference we make betwixt them, in this respect, arises not from the mere perception. So strong is the prejudice for the distinct continued existence of the former qualities, that when the contrary opinion is advanced by modern philosophers, people imagine they can almost refute it from their feeling and experience, and that their very senses contradict this philosophy. 'Tis also evident, that colours, sounds, &c. are originally on the same footing with the pain that arises from steel, and pleasure that proceeds from a fire; and that the difference betwixt them is founded neither on perception nor reason, but on the imagination. For as they are confessed to be, both of them, nothing but perceptions arising from the particular configurations and motions of the parts of body, wherein possibly can their difference consist? Upon the whole, then, we may conclude, that, as far as the senses are judges, all perceptions are the same in the manner of their existence.
We may also observe, in this instance of sounds and colours, that we can attribute a distinct continued existence to objects without ever consulting _reason_, or weighing our opinions by any philosophical principles. And, indeed, whatever convincing arguments philosophers may fancy they can produce to establish the belief of objects independent of the mind, 'tis obvious these arguments are known but to very few; and that 'tis not by them that children, peasants, and the greatest part of mankind, are induced to attribute objects to some impressions, and deny them to others. Accordingly, we find, that all the conclusions which the vulgar form on this head, are directly contrary to those which are confirmed by philosophy. For philosophy informs us, that every thing which appears to the mind, is nothing but a perception, and is interrupted and dependent on the mind; whereas the vulgar confound perceptions and objects, and attribute a distinct continued existence to the very things they feel or see. This sentiment, then, as it is entirely unreasonable, must proceed from some other faculty than the understanding. To which we may add, that, as long as we take our perceptions and objects to be the same, we can never infer the existence of the one from that of the other, nor form any argument from the relation of cause and effect; which is the only one that can assure us of matter of fact. Even after we distinguish our perceptions from our objects, 'twill appear presently that we are still incapable of reasoning from the existence of one to that of the other: so that, upon the whole, our reason neither does, nor is it possible it ever should, upon any supposition, give us an assurance of the continued and distinct existence of body. That opinion must be entirely owing to the _imagination_: which must now be the subject of our inquiry.
Since all impressions are internal and perishing existences, and appear as such, the notion of their distinct and continued existence must arise from a concurrence of some of their qualities with the qualities of the imagination; and since this notion does not extend to all of them, it must arise from certain qualities peculiar to some impressions. 'Twill, therefore, be easy for us to discover these qualities by a comparison of the impressions, to which we attribute a distinct and continued existence, with those which we regard as internal and perishing. We may observe, then, that 'tis neither upon account of the involuntariness of certain impressions, as is commonly supposed, nor of their superior force and violence, that we attribute to them a reality and continued existence, which we refuse to others that are voluntary or feeble. For 'tis evident, our pains and pleasures, our passions and affections, which we never suppose to have any existence beyond our perception, operate with greater violence, and are equally involuntary, as the impressions of figure and extension, colour and sound, which we suppose to be permanent beings. The heat of a fire, when moderate, is supposed to exist in the fire; but the pain which it causes upon a near approach is not taken to have any being except in the perception. These vulgar opinions, then, being rejected, we must search for some other hypothesis, by which we may discover those peculiar qualities in our impressions, which makes us attribute to them a distinct and continued existence.
After a little examination, we shall find, that all those objects, to which we attribute a continued existence, have a peculiar _constancy_, which distinguishes them from the impressions whose existence depends upon our perception. Those mountains, and houses, and trees, which lie at present under my eye, have always appeared to me in the same order; and when I lose sight of them by shutting my eyes or turning my head, I soon after find them return upon me without the least alteration. My bed and table, my books and papers, present themselves in the same uniform manner, and change not upon account of any interruption in my seeing or perceiving them. This is the case with all the impressions, whose objects are supposed to have an external existence; and is the case with no other impressions, whether gentle or violent, voluntary or involuntary. This constancy, however, is not so perfect as not to admit of very considerable exceptions. Bodies often change their position and qualities, and, after a little absence or interruption, may become hardly knowable. But here 'tis observable, that even in these changes they preserve a _coherence_, and have a regular dependence on each other; which is the foundation of a kind of reasoning from causation, and produces the opinion of their continued existence. When I return to my chamber after an hour's absence, I find not my fire in the same situation in which I left it; but then I am accustomed, in other instances, to see a like alteration produced in a like time, whether I am present or absent, near or remote. This coherence, therefore, in their changes, is one of the characteristics of external objects, as well as their constancy.
Having found that the opinion of the continued existence of body depends on the _coherence_ and _constancy_ of certain impressions, I now proceed to examine after what manner these qualities give rise to so extraordinary an opinion. To begin with the coherence; we may observe, that though those internal impressions, which we regard as fleeting and perishing, have also a certain coherence or regularity in their appearances, yet 'tis of somewhat a different nature from that which we discover in bodies. Our passions are found by experience to have a mutual connexion with, and dependence on each other; but on no occasion is it necessary to suppose that they have existed and operated, when they were not perceived, in order to preserve the same dependence and connexion, of which we have had experience. The case is not the same with relation to external objects. Those require a continued existence, or otherwise lose, in a great measure, the regularity of their operation. I am here seated in my chamber, with my face to the fire; and all the objects that strike my senses are contained in a few yards around me.
My memory, indeed, informs me of the existence of many objects; but, then, this information extends not beyond their past existence, nor do either my senses or memory give any testimony to the continuance of their being. When, therefore, I am thus seated, and revolve over these thoughts, I hear on a sudden a noise as of a door turning upon its hinges; and a little after see a porter, who advances towards me. This gives occasion to many new reflections and reasonings. First, I never have observed that this noise could proceed from any thing but the motion of a door; and therefore conclude, that the present phenomenon is a contradiction to all past experience, unless the door, which I remember on t'other side the chamber, be still in being. Again, I have always found, that a human body was possessed of a quality which I call gravity, and which hinders it from mounting in the air, as this porter must have done to arrive at my chamber, unless the stairs I remember be not annihilated by my absence. But this is not all. I receive a letter, which, upon opening it, I perceive by the handwriting and subscription to have come from a friend, who says he is two hundred leagues distant.
'Tis evident I can never account for this phenomenon, conformable to my experience in other instances, without spreading out in my mind the whole sea and continent between us, and supposing the effects and continued existence of posts and ferries, according to my memory and observation. To consider these phenomena of the porter and letter in a certain light, they are contradictions to common experience, and may be regarded as objections to those maxims which we form concerning the connexions of causes and effects. I am accustomed to hear such a sound, and see such an object in motion at the same time. I have not received, in this particular instance, both these perceptions. These observations are contrary, unless I suppose that the door still remains, and that it was opened without my perceiving it: and this supposition, which was at first entirely arbitrary and hypothetical, acquires a force and evidence by its being the only one upon which I can reconcile these contradictions. There is scarce a moment of my life, wherein there is not a similar instance presented to me, and I have not occasion to suppose the continued existence of objects, in order to connect their past and present appearances, and give them such an union with each other, as I have found, by experience, to be suitable to their particular natures and circumstances. Here, then, I am naturally led to regard the world as something real and durable, and as preserving its existence, even when it is no longer present to my perception.
But, though this conclusion, from the coherence of appearances, may seem to be of the same nature with our reasonings concerning causes and effects, as being derived from custom, and regulated by past experience, we shall find, upon examination, that they are at the bottom considerably different from each other, and that this inference arises from the understanding and from custom, in an indirect and oblique manner. For 'twill readily be allowed, that since nothing is ever really present to the mind, besides its own perceptions, 'tis not only impossible that any habit should ever be acquired otherwise than by the regular succession of these perceptions, but also that any habit should ever exceed that degree of regularity. Any degree, therefore, of regularity in our perceptions, can never be a foundation for us to infer a greater degree of regularity in some objects which are not perceived, since this supposes a contradiction, viz. a habit acquired by what was never present to the mind. But, 'tis evident that, whenever we infer the continued existence of the objects of sense from their coherence, and the frequency of their union, 'tis in order to bestow on the objects a greater regularity than what is observed in our mere perceptions. We remark a connexion betwixt two kinds of objects in their past appearance to the senses, but are not able to observe this connexion to be perfectly constant, since the turning about of our head, or the shutting of our eyes, is able to break it. What, then, do we suppose in this case, but that these objects still continue their usual connexion, notwithstanding their apparent interruption, and that the irregular appearances are joined by something of which we are insensible? But as all reasoning concerning matters of fact arises only from custom, and custom can only be the effect of repeated perceptions, the extending of custom and reasoning beyond the perceptions can never be the direct and natural effect of the constant repetition and connexion, but must arise from the cooperation of some other principles.
I have already observed,[3] in examining the foundation of mathematics, that the imagination, when set into any train of thinking, is apt to continue even when its object fails it, and, like a galley put in motion by the oars, carries on its course without any new impulse. This I have assigned for the reason, why, after considering several loose standards of equality, and correcting them by each other, we proceed to imagine so correct and exact a standard of that relation as is not liable to the least error or variation. The same principle makes us easily entertain this opinion of the continued existence of body. Objects have a certain coherence even as they appear to our senses; but this coherence is much greater and more uniform if we suppose the objects to have a continued existence; and as the mind is once in the train of observing an uniformity among objects, it naturally continues till it renders the uniformity as complete as possible. The simple supposition of their continued existence suffices for this purpose, and gives us a notion of a much greater regularity among objects, than what they have when we look no farther than our senses.
But whatever force we may ascribe to this principle, I am afraid 'tis too weak to support alone so vast an edifice as is that of the continued existence of all external bodies; and that we must join the _constancy_ of their appearance to the _coherence_, in order to give a satisfactory account of that opinion. As the explication of this will lead me into a considerable compass of very profound reasoning, I think it proper, in order to avoid confusion, to give a short sketch or abridgment of my system, and afterwards draw out all its parts in their full compass. This inference from the constancy of our perceptions, like the precedent from their coherence, gives rise to the opinion of the _continued_ existence of body, which is prior to that of its _distinct_ existence, and produces that latter principle.
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