once the parallel is obvious between these and the functions commonly assigned to pleasure and to pain. But, though our creature should physically experience these contrasting functions, could it mentally experience any corresponding contrasts in terms of a single sense? I answer, "Yes"; and my grounds for this answer have, as I believe, close relationship with those facts which underlie the mental experiences commonly termed desire and aversion, — experiences by no means so coincident with pleasures and pains as is commonly conceived. It will prove of importance to our subject to clear up this matter.
I desire a horse. Here the mental image of the horse is not alone the object of the desire. This concept-object also contains ideas, more or less abstract, of somewhat that might be experienced in connection with the horse. Precisely what the somewhat is that must be added to constitute my desire would be differently stated by various authors. Some contend that this mere unobstructed dwelling-in-the-mind of any pleasant concept constitutes desire; others, that an unobstructed idea of 'mine' must be added. Others would hold that an unobstructed idea, however abstract, of 'exerting myself to realize, or to continue the concept,' or of 'making it mine,' must be contained in desire. By "unobstructed" in all these cases I mean "uninterrupted by the idea of my resisting this realization."
Now, I maintain of any mental state that it is its tendency toward an act calculated to reproduce its object, that constitutes its essential characteristic as a desire, if it be such. As we shall show, ideas of pleasant experiences, from reasons lying within the conditions of their biological development, usually do tend strongly toward acts that should reproduce their object. Here the proper tendency is emphatically innate within the idea of the pleasure alone. This is why the first class of authors are led to contend that the mere dwelling-in-the-mind of pleasure concepts is desiring. Some ideas of past experiences do not possess this tendency within themselves; therefore, concepts containing such ideas must, in order to constitute desires, also contain somewhat further that does possess such a tendency. My
