The Principles of Psychology, Volume 1 · William James
ARE ALL DIFFERENCES DIFFERENCES OF COMPOSITION?
_A_ = _X_ + _d_; _B_ = (_X_ + _d_) + _d_, or _X_ + 2_d_; _C_ = _X_ + 3_d_; _D_ = _X_ + 4_d_; ........ If _X_ itself were ultimately composed of _d_'s we should have the entire series explained as due to the varying combination and re-combination with itself of an unvarying element; and all the apparent differences of quality would be translated into differences of quantity alone. This is the sort of reduction which the atomic theory in physics and the mind-stuff theory in psychology regard as their ideal. So that, following the analogy of our instances, one might easily be tempted to generalize and to say that all difference is but addition and subtraction, and that what we called 'differential' discrimination is only 'existential' discrimination in disguise; that is to say, that where _A_ and _B_ differ, we merely discern something in the one which the other is without. _Absolute identity in things up to a certain point, then absolute non-identity,_ would on this theory take the place of those ultimate qualitative unlikenesses between them, in which we naturally believe; and the mental function of discrimination, ceasing to be regarded as an ultimate one, would resolve itself into mere logical affirmation and negation, or perception that a feature found in one thing, in another does not exist.
To me it seems much wiser to turn away from such transcendental extravagances of speculation, and to abide by the natural appearances. These would leave unlikeness as an indecomposable relation amongst things, and a relation moreover of which there were all degrees. Absolute not-sameness would be the maximal degree, absolute sameness the minimal degree of this unlikeness, the discernment of which would be one of our ultimate cognitive powers.[411] Certainly the natural appearances are dead against the notion that no qualitative differences exist. With the same clearness with which, in certain objects, we do feel a difference to be a mere matter of _plus_ and _minus_, in other objects we feel that this is not the case. Contrast our feeling of the difference between the length of two lines with our feeling of the difference between blue and yellow, or with that between right and left. Is right equal to left with something added? Is blue yellow _plus_ something? If so, _plus_ what?[412] So long as we stick to _verifiable_ psychology, _we are forced to admit that differences of simple_ KIND _form an irreducible sort of relation_ between some of the elements of our experience, and forced to deny that differential discrimination can everywhere be reduced to the mere ascertainment that elements present in one fact, in another fail to exist. The perception that an element exists in one thing and does not exist in another and the perception of qualitative difference are, in short, entirely disconnected mental functions.[413]
But at the same time that we insist on this, we must also admit that differences of quality, however abundant, are not the only distinctions with which our mind has to deal. Differences which seem of mere composition, of number, of _plus_ and _minus_, also abound.[414] But it will be best for the present to disregard all these quantitative cases and, taking the others (which, by the least favorable calculation, will still be numerous enough), to consider next _the manner in which we come to cognize simple differences of kind_. We cannot _explain_ the cognition; we can only ascertain the conditions by virtue of which it occurs.
Text from Project Gutenberg, public domain in the United States.