A Treatise of Human Nature · Book I · Part IV · Section II (2/5)

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 it is 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 it is of somewhat a different nature, from that which we discover in bodies. Our passions are found by experience to have a mutual connection 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 connection, 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 anything 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 the 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. It is 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 connections 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 it will readily be allowed, that since nothing is ever really present to the mind, besides its own perceptions, it is 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 it is evident, that whenever we infer the continued existence of the objects of sense from their coherence, and the frequency of their union, it is in order to bestow on the objects a greater regularity than what is observed in our mere perceptions. We remark a connection betwixt two kinds of objects in their past appearance to the senses, but are not able to observe this connection 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 connection, 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 connection, but must arise from the cooperation of some other principles.

I have already observed,36 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 object.% 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 it is 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.

When we have been accustomed to observe a constancy in certain impressions, and have found, that the perception of the sun or ocean, for instance, returns upon us after an absence or annihilation with like parts and in a like order, as at its first appearance, we are not apt to regard these interrupted perceptions as different, (which they really are) but on the contrary consider them as individually the same, upon account of their resemblance. But as this interruption of their existence is contrary to their perfect identity, and makes us regard the first impression as annihilated, and the second as newly created, we find ourselves somewhat at a loss, and are involved in a kind of contradiction. In order to free ourselves from this difficulty, we disguise, as much as possible, the interruption, or rather remove it entirely, by supposing that these interrupted perceptions are connected by a real existence, of which we are insensible. This supposition, or idea of continued existence, acquires a force and vivacity from the memory of these broken impressions, and from that propensity, which they give us, to suppose them the same; and according to the precedent reasoning, the very essence of belief consists in the force and vivacity of the conception.

In order to justify this system, there are four things requisite. First, To explain the principium individuationis, or principle of identity. Secondly, Give a reason, why the resemblance of our broken and interrupted perceptions induces us to attribute an identity to them. Thirdly, Account for that propensity, which this illusion gives, to unite these broken appearances by a continued existence. Fourthly and lastly, Explain that force and vivacity of conception, which arises from the propensity.

First, As to the principle of individuation; we may observe, that the view of any one object is not sufficient to convey the idea of identity. For in that proposition, an object is the same with itself, if the idea expressed by the word, object, were no ways distinguished from that meant by itself; we really should mean nothing, nor would the proposition contain a predicate and a subject, which however are implied in this affirmation. One single object conveys the idea of unity, not that of identity.

On the other hand, a multiplicity of objects can never convey this idea, however resembling they may be supposed. The mind always pronounces the one not to be the other, and considers them as forming two, three, or any determinate number of objects, whose existences are entirely distinct and independent.

Since then both number and unity are incompatible with the relation of identity, it must lie in something that is neither of them. But to tell the truth, at first sight this seems utterly impossible. Betwixt unity and number there can be no medium; no more than betwixt existence and nonexistence. After one object is supposed to exist, we must either suppose another also to exist; in which case we have the idea of number: Or we must suppose it not to exist; in which case the first object remains at unity.

To remove this difficulty, let us have recourse to the idea of time or duration. I have already observed,37 that time, in a strict sense, implies succession, and that when we apply its idea to any unchangeable object, it is only by a fiction of the imagination, by which the unchangeable object is supposed to participate of the changes of the coexistent objects, and in particular of that of our perceptions. This fiction of the imagination almost universally takes place; and it is by means of it, that a single object, placed before us, and surveyed for any time without our discovering in it any interruption or variation, is able to give us a notion of identity. For when we consider any two points of this time, we may place them in different lights: We may either survey them at the very same instant; in which case they give us the idea of number, both by themselves and by the object; which must be multiplied, in order to be conceived at once, as existent in these two different points of time: Or on the other hand, we may trace the succession of time by a like succession of ideas, and conceiving first one moment, along with the object then existent, imagine afterwards a change in the time without any variation or interruption in the object; in which case it gives us the idea of unity. Here then is an idea, which is a medium betwixt unity and number; or more properly speaking, is either of them, according to the view, in which we take it: And this idea we call that of identity. We cannot, in any propriety of speech, say, that an object is the same with itself, unless we mean, that the object existent at one time is the same with itself existent at another. By this means we make a difference, betwixt the idea meant by the word, object, and that meant by itself, without going the length of number, and at the same time without restraining ourselves to a strict and absolute unity.

Thus the principle of individuation is nothing but the invariableness and uninterruptedness of any object, through a supposed variation of time, by which the mind can trace it in the different periods of its existence, without any break of the view, and without being obliged to form the idea of multiplicity or number.

I now proceed to explain the second part of my system, and show why the constancy of our perceptions makes us ascribe to them a perfect numerical identity, though there be very long intervals betwixt their appearance, and they have only one of the essential qualities of identity, viz. invariableness. That I may avoid all ambiguity and confusion on this head, I shall observe, that I here account for the opinions and belief of the vulgar with regard to the existence of body; and therefore must entirely conform myself to their manner of thinking and of expressing themselves. Now we have already observed, that however philosophers may distinguish betwixt the objects and perceptions of the senses; which they suppose coexistent and resembling; yet this is a distinction, which is not comprehended by the generality of mankind, who as they perceive only one being, can never assent to the opinion of a double existence and representation. Those very sensations, which enter by the eye or ear, are with them the true objects, nor can they readily conceive that this pen or paper, which is immediately perceived, represents another, which is different from, but resembling it. In order, therefore, to accommodate myself to their notions, I shall at first suppose; that there is only a single existence, which I shall call indifferently object or perception, according as it shall seem best to suit my purpose, understanding by both of them what any common man means by a hat, or shoe, or stone, or any other impression, conveyed to him by his senses. I shall be sure to give warning, when I return to a more philosophical way of speaking and thinking.