All such explicit grasp of relation involves a new direction of adjustive effort, or of (volitional) attention. Just as the analytic resolution of a complex demands a special effort in the way of limited concentration and resistance to irrelevant concomitants, so the comparison of two presentations in order to discern their relation imposes a further special task in the shape of a comprehensive grasp. The special difficulties of the process are manifest. Comparative attention to two presentations, say two colors in local, or two tones in temporal juxtaposition is not merely the carrying out of a simple adjustive process in one direction only, but the carrying out of a double and yet co-ordinated adjustive process.
The fact that there is a general tendency to simple modes of adjustment subserving a comparatively simple structure or pattern of consciousness, and the fact that complex simultaneous adjustments, as in the case of doing different things at the same time, and in that of the synthetic relating process of thought, are rare and acquired with difficulty, suggest that a special nervous process is involved, consisting of a double and divergent stream of innervation, each branch of which has to be kept going in certain relations of time, as also of proportionate strength, with the other branch.
The process of synthetic or relating activity just described may take the direction of consciously grasping the relations immediately presented along with presentation, and more particularly the co-existence of attributes in the same object, and the space and time relations of presentations. To note the juxtaposition of yellow and white in a daisy or the co-existence of its form and color, or the spatial inclusion of its yellow centre in an extended whole, is evidently to discern relations and so to carry out a process of conscious synthesis.
It is however in discerning the most comprehensive relations of likeness and unlikeness that thought shows itself most clearly to be a synthetic process. Thinking has in a special manner to do with the detection of similarity and dissimilarity or difference. Such relating by way of difference or agreement is what we ordinarily understand by comparison.
The relations of similarity and dissimilarity as comprehensive relations connecting presentations remote as well as proximate in time are spoken of as internal and thus marked off from the external relations of time and place. It is true as we have just seen that they are involved along with the latter. Thus in discerning the relations of the parts of an object, we must differentiate them. Yet the two modes of relating are distinct. I discriminate two colors in local juxtaposition not quâ juxtaposed but quâ different in their quality. The juxtaposition may greatly assist the discriminative process, but this circumstance does not make the juxtaposition and the qualitative difference one whit less distinct as relations.
It may be added that the greater comprehensiveness of the so-called internal relations is seen in the circumstance that the relations of time and place, just like the separate qualities or attributes of objects, are themselves modes of similarity and dissimilarity. Thus the relation of local contiguity between two elements is something common to these and other contiguous pairs. Moreover, it is evident that in such a case each element is recognised as having a different position from the other. Similarity with the temporal relations of events.
Comparison.—We may now glance at the operations here brought under the head of comparison, the bringing of different presentative or representative materials before the mind simultaneously and keeping them in consciousness in order to note their relations of similarity or dissimilarity. Here as in the case of Analysis or Abstraction we shall illustrate the process by selecting relatively simple modes of the operation carried out on immediately presented sense-material.
Likeness and Difference.—We may here assume that likeness and unlikeness are two perfectly distinct relations. To apprehend a similarity between two sensations, say tones, is an intellectual process which we all recognise as radically unlike that of apprehending a difference.
Yet while the consciousness of likeness and that of difference are thus radically distinct, as psychical processes, it is evident that the relations of likeness and difference are presented together in close connection. As we all know similarity discloses itself in the midst of difference. This is obvious in the case of all complex presentations, as when we assimilate two objects on the ground of a color resemblance. Not only so, since even in the case of sensation-elements (e. g. color-sensations) likeness is a thing of degree shading off from perfect likeness or indistinguishableness to just recognisable affinity, it follows that here, too, likeness and difference are given together in mutual implication.
Since resemblance and difference are thus uniformly presented together, it is to be expected that comparison will commonly include the two processes, assimilation and discrimination. And this is so. We see likeness amid difference, e. g. a common trait in two faces along with striking dissimilarities. On the other hand we contrast two objects in respect of some common quality as color, form, beauty and so forth, which common element constitutes the ground or fundamentum of the comparison.