Front Page Titles (by Subject) CHAPTER XXI.: THE RELATIONS OF COEXISTENCE AND NON-COEXISTENCE. - The Principles of Psychology
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CHAPTER XXI.: THE RELATIONS OF COEXISTENCE AND NON-COEXISTENCE. - Herbert Spencer, The Principles of Psychology 
The Principles of Psychology (London: Longman, Brown, Green and Longmans, 1855).
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THE RELATIONS OF COEXISTENCE AND NON-COEXISTENCE.
§ 89. It is tolerably evident, even à priori, that, simple as it seems, the relation of coexistence is in reality compound. Though, in the adult mind, apparently undecomposable, yet it is a corollary from very obvious truths, that this relation is originally synthetic. For as coexistence implies two things; as, further, the two things which coexist, cannot occupy consciousness at the same instant; and as they cannot pass through consciousness in simple succession—seeing that they would then be known as sequent and not coexistent—it follows that coexistence can be disclosed only by some duplex act of thought. It is true that the two terms of a relation of coexistence—as the ends of a line at which we look, or the opposite sides of a stick which we grasp—ordinarily appear to be known, not in two states of consciousness, but in one. But it needs only to call to mind the extremely complex process by which our perceptions of objects are built up; and to remember that what in the infant is an elaborate synthesis, afterwards becomes an instantaneous and, as it would seem, direct cognition; to see that no apparent simultaneity in the consciousness of the two things between which there is a relation of coexistence, can be taken as disproving their original seriality. Leaving general considerations however, let us look at the matter more nearly.
If the eyes be directed to two small dots placed close together upon a sheet of paper, the facts that there are two, that they coexist, and that there is a certain space between them, certainly appear to be given in the same immediate intuition: and it seems a scarcely credible proposition that by a nascent intelligence they can neither be known as two, nor as coexistent, nor as having relative positions. But on re-reading § 58 it will, I think, become clear that at first, any two such dots can produce nothing but an indefinite visual sensation, as simple as one of sound or smell. For as was shown, the possibility of distinguishing the image upon the retina as consisting of not one impression, but of two, implies in the first place, that the retina consists of parts capable of being separately excited; seeing that were it but the expansion of one nerve, the stimulation of any part would produce the same effect upon consciousness, while the stimulation of two or more parts could do nothing but increase the intensity of the sensation. And it implies in the second place, that the separate stimulations of these separate parts are distinguishable from one another by consciousness; seeing that did they all produce one effect on consciousness, the result would be the same as though they were one. But before the separate stimulations of these separate parts can be distinguished from one another by consciousness, there must be some experiences. For the two parts of the retina simultaneously affected by the images of two points, to be known as yielding two sensations and not one sensation, implies a knowledge of the parts as separate; and to suppose that this can exist anterior to experience is absurd. Or to state the case more conclusively:—Coexistence being unthinkable without a space in which the things may coexist, it follows that the two points described, cannot be known as coexistent without being also known as out of each other—as at some distance from each other. But, as before explained, to suppose that when two sentient points on the surface of the organism are first simultaneously stimulated, some particular distance is thereby suggested, is to fall into the absurdity of supposing that an idea of some particular distance already exists in the mind (§ 58). Evidently then, as by a nascent intelligence, the space between the two coexistent points is incognizable; and as their coexistence cannot be otherwise conceived, it follows that at first they cannot be known as coexistent.
From all which it is an obvious corollary, that the relation of coexistence is disclosed by the same experiences that disclose extension. But now we have to observe concerning these experiences, a fact not before noticed. The repeatedly described consolidation of serial states of consciousness into quasi single states, is not the whole of the process by which the ideas of coexistence and extension are evolved. It is the peculiarity alike of every tactual and visual series which enters into the genesis of these ideas, that not only does it admit of being transformed into a composite state, in which the successive positions become simultaneous positions, but it admits of being reversed. The chain of states of consciousness, A to Z, produced by the motion of a limb, or of something over the skin, or of the eye along the outline of an object, may with equal facility be gone through from Z to A. Unlike those states of consciousness constituting our perception of sequence, which do not admit of an unresisted change in their order, those which constitute our perception of coexistence admit of their order being inverted—occur as readily in one direction as the other. And this is the especial experience by which the relation of coexistence is disclosed. Let us glance at the chief phases of this experience.
Recurring to the adjacent dots, it will be observed on experiment, that though very close and very small, they can never be both perfectly present to consciousness at the same time. The one on which, at any moment, the visual axes converge, is alone perceived with complete distinctness. The other, though, as it would at first seem, very clearly before the mind, cannot be perceived with the highest degree of definiteness until the visual axes converge upon it; and when the gaze is thus transferred, the dot first contemplated ceases to be so definitely perceived. Moreover, if, while the eyes are fixed upon one of the dots, the thoughts are directed to the other, it will be found that in proportion as the other is distinctly thought of, the one to which the eyes are directed tends to lapse out of consciousness. Both which facts go to show, alike that the serial experiences which originally gave the knowledge of coexistent positions, never wholly cease to be used; and that, even under the most favourable circumstances, the two terms of a relation of coexistence are not present to the mind with equal distinctness; but that while the one is clearly before consciousness, the other is nascent in a higher or lower degree. Let us now observe what happens when the dots are further apart. If they are extremely minute, it will be found that even at the distance of an inch apart, the one is invisible when the eyes are directed to the other, and cannot be known as coexistent with it except by a definite transfer of the attention. If they are dots of moderate size, the consciousness of one will be accompanied by some consciousness of the other until they are separated by a space of six or eight inches; beyond which, this nascent consciousness wholly ceases. With still larger objects, there must be a still larger interval—or, more strictly speaking, a still greater subtended angle—to produce the same result. But however large the objects, it will be found that there is a distance at which either ceases to be in any degree presented to the mind, when the eyes are directed to the other. The unregarded object, when gradually removed to the outskirts of the field of view, does not disappear suddenly; but fades into nothingness so gradually that it is impossible to say when the nascent consciousness of it wholly ceases. And as, between those relative positions in which the coexistence of two objects can be known only by a slight turn of the head, and those in which it can be known only by turning the head half round, there is also a series of imperceptible transitions; it follows that the coexistence of two dots lying close together, and that of two objects lying respectively behind and before the observer, are known in modes which, however apparently different, are united by insensible gradations, and must be primordially the same. In both cases, the terms of the relation of coexistence cannot be perfectly present to consciousness at the same moment. In both cases, motion is required to bring that term of the relation of which there is either no consciousness or but imperfect consciousness, distinctly before the mind. And the differences are simply between the degrees of motion, and between the degrees in which the consciousness is nascent.
This being understood, let us consider in what way we can know the coexistence of two things not visible together. When an adult, having just seen some object A, immediately after sees another object B, he usually asserts their coexistence on the strength of this single observation. He is manifestly enabled to do this by an accumulation of previous experiences; from which he has drawn the induction that certain groups of phenomena are persistent. But what does he mean by persistent? He means that the phenomena are of a kind which he can again become conscious of with the same vividness as before. He means that on turning round his head, the object A, will again impress him as it did at first. The entire contents of his assertion that A and B coexist, is, that the states of consciousness which they severally produce in him, can be alternated as often as he pleases. Leaving, however, the coexistence that is known inferentially, we must here concern ourselves with those primordial experiences which first disclose it. By an incipient intelligence, the impressions produced by the two things A and B, seen in succession, cannot be known to differ in their persistence from two sounds heard one after the other. In either case, there is nothing but a sequence of states of consciousness. How then, does the one relation come to be distinguished from the other? Simply by finding that whereas the terms of the second sequence cannot be known in the reverse order with equal vividness, those of the other can. It is perpetually found that while certain states of consciousness follow one another with as much facility and clearness in one direction as in the opposite (A, B—B, A) others do not; and hence results a differentiation of the relation of coexistence from that of sequence. And not only is it that coexistence is originally thus known; but, as just pointed out, it is that, subjectively considered, our whole knowledge of the relation of coexistence consists in recognizing the equal facility with which the terms of the relation will pass through consciousness in either order.
Still more manifest will this become, when it is observed that there are coexistences which even the adult never knows otherwise than through this test. Now that I am writing, I feel in my foot the warmth of the fire; I am further aware of the pressure of my arm upon the desk, and my back against the chair; I see the paper on which I write; and I hear a rumble in the street. I find it quite impossible, however, to think of all these things at the same instant: I cannot unite the heat, the sound, the pressure, and the whiteness, in the same state of consciousness. How then do I know that I am receiving these various impressions at one time? How do I know that the external objects producing them are coexistent? Simply from the fact that I can be successively conscious of these various feelings in any order with equal facility. And could I not do this, I should not know the corresponding phenomena as coexistent.
§ 90. The equal facility with which the terms of a relation of coexistence can be thought of in either order, is evidently knowable by us simply through an internal feeling. That we habitually notice the feelings accompanying changes in consciousness, is proved by the fact that we distinguish them by words. When we speak of a thing as hard to think, or easy to believe, we express by these adverbs the presence or absence of a certain mental tension. In the one case, the antecedent and consequent of the thought can be made to follow only by a great effort; in the other, by little or no effort. When attempting to remember a name we have forgotten; or when forcing ourselves to reflect on some subject to which we are averse, or of which we are tired; or when trying to form an unusually complex conception; we are distinctly conscious of an inward strain. Whence it is clear, that the states of consciousness constituting a thought, may follow one another either with facility or with any degree of difficulty; and that the facility or difficulty of a transition is known to us by its accompanying sensation.
Hence then, when it is said that the relation of coexistence is one of which the terms will follow one another through consciousness in either order with equal facility, the thing asserted is, a likeness or equality of the two feelings which accompany respectively, the change from antecedent to consequent, and the change from consequent to antecedent. Not a likeness or equality of the two feelings produced by the contrasts of the terms; for these must differ according to the order in which the terms are contemplated; but a likeness or equality of the two feelings of resistance—or rather in this case, non-resistance—which occur at the moments of transition.
So that the relation of coexistence is to be defined as a union of two relations of sequence, such that while the terms of the one are exactly like those of the other in kind and degree, and exactly the reverse in their order of succession, they are exactly like them in the feeling which accompanies that succession. Or otherwise, it may be defined as consisting of two changes in consciousness, which, though absolutely opposite in other respects, are perfectly alike in the absence of strain. And of course the relation of non-coexistence differs in this, that though one of the two changes occurs without any feeling of tension, the other does not.
§ 91. It may be worth while just to point out, that these conclusions are indicated even by à priori considerations. For if, on the one hand, the great mass of outward things are statical, are persistent, are not manifesting any active change; and if, on the other hand, perpetual change is the law of the inner world—is the primary condition under which only consciousness can continue; there arises the question—How can the outer statical phenomena, be ever represented by the inner dynamical phenomena? How can the no-changes outside, ever be symbolized by the changes inside? That changes in the non-ego may be expressed by changes in the ego, is comprehensible enough; but how is it possible that objective rest, can be signified by subjective motion? Evidently there is only one possibility. A consciousness ever in a state of change, can represent to itself a no-change, only by an inversion of one of its changes—by a duplication of consciousness equivalent to an arrest—by a regress which undoes a previous progress—by two changes which exactly neutralize each other.
Finally, the reader should be reminded that this analysis of the relation of coexistence, resulting as it does in the conclusion that it is a relation disclosed by experience, supplies the ultimate disproof of the hypothesis that Space is a form of thought; seeing that the cognition of coexistence is the primitive element out of which the cognition of space is built—is the element without which even the germ of that cognition is impossible.