70 GENIUS AND MENTAL DISEASE. 
These false perceptions, these illusions and hallucina- 
tions, while they do not necessarily indicate any mental 
unsoundness, have been, however, the fruitful source of 
those apparitions, whether of demons, fairies, or ghosts, 
which have added-to the credulity of man, intensified 
his superstitions, and made possible the organization of 
human error under such forms of belief as are typically 
illustrated by witchcraft and spiritualism. 
So long as an individual is conscious that the illusions 
and hallucinations of his senses are unreal—merely 
*‘such stuff as dreams are made of’’—the intellect is 
not affected ; but when the false perceptions are ac- 
cepted as realities, the mind itself is then involved, and 
a delusion or a false belief is said to exist. 
A delusion may be based upon false perceptions ; 
faulty ideas from perverted reasoning about real events, 
or from mental inability to distinguish differences in 
things. 
A false belief is not, however, of itself indicative of 
insanity, so long as it is in harmony with the individual’s 
common mode of thought and with the spirit of the age. 
This is apparent when it is remembered that witchcraft 
—now regarded as a delusion—was, not long since, held 
to be a truth; indeed, such master-minds as Bacon, 
Jewel, Luther, Calvin, Wesley, Blackstone, Coke, and 
Dr. Johnson, in accepting as a truth that which we now 
know was a mental epidemic of error, reflected only the 
universal belief of the age, and were free from any taint 
of insanity. 
That the standard of mental health is variable because 
it is conditioned by race, age, environment, and circum- 
stances, is abundantly attested by the history of the 
past ; and this fact should be recalled in discussing the 
kinship of genius and madness. 
The popular literature relating to genius and insanity 
is so meager and fragmentary that the recent contribu- 
