From this point of view the peculiar characteristics of the impressions of sense are due to some peculiarities in the cortical processes which are their physical bases—peculiarities which are usually due to the action of a peripheral current. Thus it may be that the sensation is more intense because the current acts upon the stored-up energy of the, cortical cells much as a spark acts upon gunpowder. If precisely the same kind of a cortical process could be induced in any other way than by the action of a peripheral current, we would presumably have an imitation sensation. There appears no good reason why there should not be many other kinds of cortical processes intermediate between those that underlie ideas and those that underlie sense-impressions, and to them mental states should correspond which are betwixt and between—neither fish, flesh, nor fowl.
Now, of the cortical processes we know nothing; I use them merely as symbols for mental facts. But the mental states we directly know, and it is quite certain that many different types of them exist, roughly corresponding to what we would expect if the above conception were true. We know that in different individuals ideas vary much in their clearness and in the degree to which they approach sensations. In the same individual they occasionally assume a form which is to him almost like a glimpse into a new world of experience. My own visual ideas, for example, are very vague and dim, and I shall never forget the two or three occasions in my life when they have for a while been vivid and brightly colored, somewhat as my visual sensations are. And occasionally we meet with experiences which are certainly originated largely or entirely from within and must be classed as ideas and yet resemble sensations so closely that they can be discriminated from them only upon reflection. These are what we term illusions and hallucinations; the other types, which we never mistake for realities, although they resemble sensations so closely, are termed pseudo-hallucinations. By the level or grade of a mental state I mean the degree to which it approximates that fullest and most perfect form of being which we find in the sense-impression, and by development I mean the process of becoming more like the sense-impression.
What can cause development? Well, in the first place, it can be caused in some individuals by concentration of attention. Most of my readers have heard the story of the painter who said he could at any time see again a sitter by looking at the chair in which he had once sat. I have met many such persons. Sometimes the process is slow and its several stages can be traced. Miss Z——, for example, after fixing her thoughts upon the image of a friend, sees a shadow appear before her which gradually assumes color, consistence, solidity, reality, and finally becomes the