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.