At the same time it is evident that the one process usually, if not in all cases, preponderates over the other. We are now specially interested in the likeness of two objects, say two faces, or two literary styles, the moment after, perhaps, in their differences. Accordingly we may say that comparison is the noting of likeness against a dimly apprehended background of difference, or a difference against a dimly apprehended background of similarity.
Conditions of Comparison.—Comparison whether specially directed to likeness or unlikeness has certain common conditions. As in the case of Abstraction these conditions may be divided into objective, or those involved in the nature or concomitants of the presentations considered as external objects, or objects of common perception; and subjective or those connected with the nature of the individual mind. As I have given a full account[98] of these elsewhere, I must content myself here with a general remark or two on the subject.
[98] In Mind, x. p. 489 et seqq.
Of the objective conditions the most important are the following: (a) There must be a moderate and favorable degree of strength or intensity in the presentations to be compared. We compare fairly bright colors better than very dull ones. (b) The common factor or ground of comparison must be sufficiently distinct. We cannot compare two tones in respect of pitch if this is unsteady. (c) Comparison is greatly aided by juxtaposition in space or time. Thus local proximity is a condition of a nice comparison of colors. With respect to temporal conditions it was found by Fechner and has been confirmed by others that immediate succession is more helpful to comparison than simultaneity. We compare sensations of weight, tone, etc., best of all when they are made to succeed one another.
With respect to subjective conditions, comparison will, it is obvious, be assisted by a good power of concentration. It will also be aided by a special sensibility for, and interest in, the particular sensuous material: witness the musician's comparison of tones as to pitch, purity, etc. Lastly reference may be made to special preparation or mental preadjustment. It is manifest that if we are expecting to see two things like one another we shall in general be more disposed to do so; similarly if we are on the lookout for difference.
It may be added that there is a special interest in likeness as such, and also in difference. Such interest predisposes a person to detect the one relation rather than the other. Hence the familiar observation that some people are particularly acute in seeing likenesses, e. g. in faces, whereas others are habitually more observant of differences.
Connection between Analysis and Comparison.—There is a close connection between the two directions of thought-activity just dealt with. To begin with, it has become evident that in the processes of comparison, analysis is always involved. Sometimes the analysis seems to precede the comparison, as when we are asked to compare two flowers in respect of their color. In other cases it appears rather as the result of comparison. Thus it is by successive comparisons of different members of a class of things, as flowers, that we gradually come to analyse out the common features of the group.
While comparison thus involves abstraction, abstraction even in the case of a single object may be said to involve the rudiments of comparison. Thus in analytically singling out for consideration the spherical form of a rain drop, we implicitly and subconsciously assimilate it to other previously known spherical objects. But for this vague imperfect accompaniment of assimilation, the analytic separation of the constituent would be difficult if not impossible. Such a subconscious reference to one or more similar things helps to direct the operation of analysis by intensifying and rendering prominent for the moment the particular constituent assimilated through the addition of an ideational element to the sensation.[99]
[99] This is well brought out by W. James, Principles of Psychology, i. p. 434 et seqq.
It follows that the thought-process is one process having two aspects or distinguishable factors. Either of these may become predominant according to special circumstances. In this way we obtain two varieties of operation, viz. Analysis or Abstraction, in which the recognition of likeness is subconscious, and Assimilative Comparison where the process of analysis is preliminary and subordinate to a conscious apprehension of likeness.