one another by such insensible
gradations, that it is impossible to discuss the one apart from the
other. The view of illusion which will be adopted in this work is that
it constitutes a kind of border-land between perfectly sane and vigorous
mental life and dementia.

And here at once there forces itself on our attention the question, What
exactly is to be understood by the term "illusion"? In scientific works
treating of the pathology of the subject, the word is confined to what
are specially known as illusions of the senses, that is to say, to false
or illusory perceptions. And there is very good reason for this
limitation, since such illusions of the senses are the most palpable
and striking symptoms of mental disease. In addition to this, it must be
allowed that, to the ordinary reader, the term first of all calls up
this same idea of a deception of the senses.

At the same time, popular usage has long since extended the term so as
to include under it errors which do not counterfeit actual perceptions.
We commonly speak of a man being under an illusion respecting himself
when he has a ridiculously exaggerated view of his own importance, and
in a similar way of a person being in a state of illusion with respect
to the past when, through frailty of memory, he pictures it quite
otherwise than it is certainly known to have been.

It will be found, I think, that there is a very good reason for this
popular extension of the term. The errors just alluded to have this in
common with illusions of sense, that they simulate the form of immediate
or self-evident cognition. An idea held respecting ourselves or
respecting our past history does not depend on any other piece of
knowledge; in other words, is not adopted as the result of a process of
reasoning. What I believe with reference to my past history, so far as I
can myself recall it, I believe instantaneously and immediately, without
the intervention of any premise or reason. Similarly, our notions of
ourselves are, for the most part, obtained apart from any process of
inference. The view which a man takes of his own character or claims on
society he is popularly supposed to receive intuitively by a mere act of
internal observation. Such beliefs may not, indeed, have all the
overpowering force which belongs to illusory perceptions, for the
intuition of something by the senses is commonly looked on as the most
immediate and irresistible kind of knowledge. Still, they mus