in mental development. A and B must be presented and noted as two distinct impressions before we become conscious of the relation A-B.1 An animal low down in the scale may have differentiated sensations, that is, be differentially impressed by this and that stimulus, e.g., thermal or tactile, and yet never rise to a clear consciousness of a relation of difference.2 Such an intellectual act or process of discrimination only becomes possible when sensations by repetition acquire a certain steadiness and persistence, and when attention is practised up to the point of relational or comparative attention, i.e., a simultaneous grasp of two impressions as two distinct yet related impressions.3

A

True discrimination develops by gradual stages out of the ⚫ process of differentiation just described. Thus we may suppose that a strong stimulating sound or light at the moment of its introduction is attended by a vague consciousness of change or transition and this supplies the germ of discrimination. child experiencing the change from darkness to light, from cold to heat, could hardly fail to note the change as such. This, however, is still a long way from a clear grasp of a precise relation of difference as defined above. The rapid disappearance of the receding experience under the superior interest of the new one would prevent the infant mind from attending to the two in their relation.

A more favourable situation would be the simultaneous presentation of two strong and widely-contrasting sensations, as two touches when the child happens to encounter two unlike substances with the two hands or two contrasting colours in juxtaposition. Here we may mark off two distinguishable stages in the development of a clear consciousness of difference. First of all, there is a vague sense of two different sensations, in which the apprehension of numerical distinction is uppermost, and that of a particular qualitative difference is indis

1 This applies to all intellection as a relational and relating process. The mental apprehension of a relation of difference, likeness, or succession in time must be carefully distinguished from the experience of having two unlike, like or successive impressions. (Cf. Ward, loc. cit., p. 45; and Lotze, Metaphysic, P. 470 f.)

2 Cf. Romanes, Mental Evolution in Animals, chap. i.

3 Cf. above, p. 158 f.

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tinct. This is illustrated by certain experiments in which the subject can say he hears two tones but does not know which is the higher. Secondly, there comes the clearer consciousness of a particular kind and amount of difference (e.g., so much pitchinterval between two tones). This last clear grasp of the relation of difference, as such, is the work of comparison, and will be explained more fully later on.2

§ 4. Law of Change or Relativity. We have already touched on the question in what sense, and to what extent, change enters into the tissue of our mental life. Our examination into the mechanism of attention and the process of differentiation will enable us to discuss the point more fully.

That there is a meaning in saying that consciousness involves change of psychical state, or has change as a fundamental condition, is indisputable. A dead level of sensation without the least introduction of freshness or variation would be indistinguishable from sleep. As Hobbes has it, "Semper idem sentire ac non sentire ad idem revertunt". This fact of the dependence of mental life on change has been formulated under the head of the Law of Relativity.*

This law of change or variety finds its explanation in part in the very conditions of vigorous nervous action. Highly recuperated structures are capable of more vigorous function than partially fatigued and exhausted ones. Prolonged stimulation of a nervous structure is attended in certain cases at least with a falling off in the intensity of the sensations.5

1 Stumpf distinguishes between ' analysis' as apprehension of numerical difference and discrimination, op. cit., i. 108. It is evident, however (as Stumpf himself seems to allow), that there cannot in the case of simultaneous sensations be an apprehension of numerical distinction or plurality apart from some vaguely-apprehended qualitative difference. Even two similar colours are only seen as two when the sensations have distinction of local quality. In the case of similar successive sensations, as tones, the difference in temporal position constitutes a kind of qualitative difference.

2 On the relation of such a clear grasp of difference to a vague sub-consciousness of difference, see James, op. cit., i. p. 526 f.

3 Cf. above, p. 156 f.

See Bain, Mental and Moral Science, p. 83; Hamilton expresses the same principle under the "Law of Variety "; see Ward, loc. cit., p. 49.

5 Stumpf points out that this decline in intensity is much more noticeable in certain classes of sensations than in others. It is hardly appreciable at all in the case of sounds. (Tonpsychologie, i. p. 18.)

Change of stimulation, on the other hand, by calling into play a fresh organ, ensures greater intensity in the psychical effect. Not only so, as was pointed out in dealing with attention, the frequent diversion of the adjustive process from one impression or region of impressions to another is necessary to a vigorous maintenance of the attention. This is strikingly illustrated in what has been called "the acquired incapacity" to attend to constant and unvarying impressions. The miller after a time fails to hear the noise of his mill.1 It is also illustrated in the fact that when we go on attending to an impression, e.g., one of bright colour, there is a falling off in intensity, which is presumably due to the slackening of the effort of attention.

This general truth has a bearing both on the intellectual processes and on the feelings. The latter is illustrated in the well-known effects of novelty, contrast, and rapid variation of impression in heightening feeling and the enjoyment of life. This effect will have to be dealt with more fully by-and-by. We are now concerned with the bearing of change or relativity on the intellectual processes.

Here the question arises, What is the precise function of change or contrast in our sensational experience? According to one rendering of the Law of Relativity, change is not only a general condition of distinct and vivid sensation, but it is one factor in determining the particular quality of a sensation. Thus it is said that black is only seen to be black in contrast to white, that the several partial colours are for us what they are because of their relations to other colours, and that blue would not be blue, green, green, and so forth, but for these relations.

That there is some meaning in this will be seen by a reference to what was said above respecting the variability of sensation, the effect of colour-contrast, etc. That one sensation may under certain circumstances modify the quality of another is certain. At the same time, this effect appears to be narrowly circumscribed. Thus Stumpf has shown that in the region of tones there is no contrast, but rather an opposite

'On such acquired inattention, see Lewes, Problems of Life and Mind, third series, i. p. 189 f.; and James, op. cit., i. p. 455 ff.

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tendency in simultaneous tones to bring one another rearer.1 These facts themselves suggest that colour-contrast is a physiological phenomenon, and does not involve a general principle of our sensational experience.

As we have seen above, the quality of a sensation is determined by its own psycho-physical process, and is only modified so far as this process is altered. The 'blueness' of a sensation of blue is thus independent of preceding sensations. Blue would still have its blueness even were the eye blind to every other colour; though, as has been pointed out, the blueness would not be so vividly and distinctly realised in this case.

All our sensational, as well as our ideational experience, is constituted by two co-operant factors or aspects. It has at once what W. James calls a "substantive" and a "transitive aspect". The sweetness of the sugar I am now tasting illustrates the former, the transition to this, say from the bitter of a medicine, illustrates the latter.

These factors are by no means equally prominent in all sensational experiences alike, or at all moments of the same experience. Thus, when I am suddenly touched, or hear a familiar sound, the transitive aspect is reduced to a vague sense of an oncoming sensation. There need not, in this case, be any awareness of a particular transition from one kind of sensation to another. Again, when a sensation, say of tone, is prolonged, I can go on within certain limits of time attending to its quality with no appreciable sense of change or transition. Here the substantive aspect is uppermost. On the other hand, in the first oncoming of a sensation after another and unlike one, as in passing out of a dark room into a bright one, the sense of change may be uppermost.

The idea that every time we have a particular sensation we are apprehending more or less distinctly its difference from other sensations leads to endless difficulties. One difficulty will occur at once. In the case of all intense sensations there seems no room for an idea of a relation of contrast. When thoroughly chilled I cannot imagine heat, and consequently cannot be said to realise cold as a contrast

1 Tonpsychologie, ii. p. 398. According to Ehrwall there are no contrast phenomena in sensations of taste.

2 See his Principles of Psychology, i. p. 243.

to hot. In the case of sensations having a number of different relations another kind of difficulty occurs. When, for example, I have a sensation of blue, as in looking at the sky, or better still, at a blue flame surrounded by darkness, what particular relation or relations of colour-difference, it may be asked, am I realising at the moment? I cannot be bringing the colour into relation to all other colours at the same moment, and yet there seems no reason why I should select any one rather than another for a particular relation of difference. It seems plain that in this case there is no clear consciousness of difference at all. The blue is differentiated, i.e., attended to as a sensation of a particular quality, but not consciously discriminated from anything in particular.

Even here, however, we can find a certain meaning in the doctrine of Relativity. Though to apprehend a sensational content as such is one thing, to note its difference from other contents another thing, this relational element is probably never wholly absent in the developed adult consciousness. Relations of contrast are among the most interesting; and, as such, are early remarked. From frequently noting them, we come in time to overlay, so to speak, our simple sensations with these relations. This applies most obviously to certain pairs of contrasting sensations, as hot cold, dark bright, hard soft, and so forth, the relation of which is specially interesting and easily noted, and generally to those opposed or contrasting experiences which underlie the particular variety of relative terms which logicians call "contraries 66 or opposites". We cannot think of hot, tall, and so forth, without having the idea of the correlative opposite, cold, short, etc., sub-excited.1 It applies also to some extent even to the series of sensations which we call the colour and tone scales. Thus a particular tone, say C1, or a particular colour, blue, is after a certain experience of the whole series to which it belongs—the eight notes of the octave, the colour-scale-apprehended vaguely at least as differing from the remaining members of the series. In other words, to use the language of the logician, the 'positives' C1, blue are accompanied by a dim, incomplete representation of the correlative negatives,' "not-C1" (i.e., tones), "not-blue" (i.e., colours). Not only so, the effect of artistic culture is undoubtedly to invest each member of the series with a network of relations of difference, or, as they have in this case been called, distance. Thus a particular colour, say blue, is attached by a particular relation of distance to green, yellow, and so forth, and so ready to suggest these. In other words, in seeing the colour we refer it to its proper place in the scale of colours, 2

(B) ASSIMILATION.

§ 5. Nature of Assimilative Process: Relation of Likeness. The second of the constituent processes entering into intel

1 On the nature of such extremes or opposites see Jevons, El. Lessons in Logic, p. 24. In the case of the sensation hot, the idea of cold would be excluded for the reason given above.

2 The law of relativity has been specially applied to the intensity of sensations by Wundt (op. cit., p. 377 ff.), who seeks to formulate a general law of relation ("Gesetz der Beziehung ") into which he incorporates the appreciation of intensity as defined by Weber's Law (cf. above, p. 89). On the whole question of relativity consult Bain, Senses and Intellect, p. 8 f. and 321 f.; and Ward, loc. cit., pp. 49, 50. The various forms of the doctrine of relativity are carefully distinguished and examined by Stumpf, Tonpsychologie, i. pp. 1-22; cf. James, op. cit., ii. 9 ff.