ILLUSIONS AND HALLUCINATIONS. 631 



From this point of view the peculiar characteristics of the im- 

 pressions 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 sen- 

 sation. 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 indi- 

 viduals 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 exam- 

 ple, 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 dis- 

 criminated 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 be- 

 coming 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. Some- 

 times 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 as- 

 sumes color, consistence, solidity, reality, and finally becomes the 



