=Discrimination defined.=--The noticing of any _part_ whatever of our object is an act of discrimination. Already on p. 218 I have described the manner in which we often spontaneously lapse into the undiscriminating state, even with regard to objects which we have already learned to distinguish. Such anæsthetics as chloroform, nitrous oxide, etc., sometimes bring about transient lapses even more total, in which numerical discrimination especially seems gone; for one sees light and hears sound, but whether one or many lights and sounds is quite impossible to tell. Where the parts of an object have already been discerned, and each made the object of a special discriminative act, we can with difficulty feel the object again in its pristine unity; and so prominent may our consciousness of its composition be, that we may hardly believe that it ever could have appeared undivided. But this is an erroneous view, the undeniable fact being that _any number of impressions, from any number of sensory sources, falling simultaneously on a mind_ WHICH HAS NOT YET EXPERIENCED THEM SEPARATELY, _will yield a single undivided object to that mind_. The law is that all things fuse that _can_ fuse, and that nothing separates except what must. What makes impressions separate is what we have to study in this chapter. =Conditions which favor Discrimination.=--I will treat successively of differences: (1) So far as they are directly _felt_; (2) So far as they are _inferred_; (3) So far as they are _singled out in compounds_. =Differences directly felt.=--The first condition is that _the things to be discriminated must_ BE _different_, either in time, place, or quality. In other words, and physiologically speaking, they must awaken neural processes which are _distinct_. But this, as we have just seen, though an indispensable condition, is not a sufficient condition. To begin with, the several neural processes must be distinct _enough_. No one can help singling out a black stripe on a white ground, or feeling the contrast between a bass note and a high one sounded immediately after it. Discrimination is here _involuntary_. But where the objective difference is less, discrimination may require considerable effort of attention to be performed at all.