Illusions | Page 4

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 must be said to come very
near illusions of sense in the degree of their self-evident certainty.
Taking this view of illusion, we may provisionally define it as any
species of error which counterfeits the form of immediate, self-evident,
or intuitive knowledge, whether as sense-perception or otherwise.
Whenever a thing is believed on its own evidence and not as a
conclusion from something else, and the thing then believed is
demonstrably wrong, there is an illusion. The term would thus appear
to cover all varieties of error which are not recognized as fallacies or
false inferences. If for the present we roughly divide all our knowledge
into the two regions of primary or intuitive, and secondary or
inferential knowledge, we see that illusion is false or spurious
knowledge of the first kind, fallacy false or spurious knowledge of the
second kind. At the same time, it is to be remembered that this division
is only a very rough one. As will appear in the course of our
investigation, the same error may be called either a fallacy or an
illusion, according as we are thinking of its original mode of production
or of the form which it finally assumes; and a thorough-going
psychological analysis of error may discover that these two classes are
at bottom very similar.
As we proceed, we shall, I think, find an ample justification for our
definition. We shall see that such illusions as those respecting ourselves
or the past arise by very much the same mental processes as those
which are discoverable in the production of illusory perceptions; and
thus a complete psychology of the one class will, at the same time,

contain the explanation of the other classes.
The reader is doubtless aware that philosophers have still further
extended the idea of illusion by seeking to bring under it beliefs which
the common sense of mankind has always adopted and never begun to
suspect. Thus, according to the idealist, the popular notion (the
existence of which Berkeley, however, denied) of an external world,
existing in itself and in no wise dependent on our perceptions of it,
resolves itself into a grand illusion of sense.
At the close of our study of illusions we shall return to this point. We
shall there inquire into the connection between those illusions which
are popularly recognized as such, and those which first come into view