Illusions | Page 6

it, present
material reality, or whether (if I may be allowed the term) he retrojects
it into the dim region of the past, and takes it for a reality that has been
he is committing substantially the same blunder. The source of the
illusion in both cases is one and the same.
It might seem to follow from this that a scientific discussion of the
subject would overlook the obvious distinction between illusions of
perception and those of memory; that it would attend simply to
differences in the mode of origination of the illusion, whatever its
external form. Our next step, then, would appear to be to determine
these differences in the mode of production.
That there are differences in the origin and source of illusion is a fact
which has been fully recognized by those writers who have made a
special study of sense-illusions. By these the term illusion is commonly
employed in a narrow, technical sense, and opposed to hallucination.

An illusion, it is said, must always have its starting-point in some actual
impression, whereas a hallucination has no such basis. Thus it is an
illusion when a man, under the action of terror, takes a stump of a tree,
whitened by the moon's rays, for a ghost. It is a hallucination when an
imaginative person so vividly pictures to himself the form of some
absent friend that, for the moment, he fancies himself actually
beholding him. Illusion is thus a partial displacement of external fact by
a fiction of the imagination, while hallucination is a total displacement.
This distinction, which has been adopted by the majority of recent
alienists[1], is a valuable one, and must not be lost sight of here. It
would seem, from a psychological point of view, to be an important
circumstance in the genesis of a false perception whether the
intellectual process sets out from within or from without. And it will be
found, moreover, that this distinction may be applied to all the varieties
of error which I propose to consider. Thus, for example, it will be seen
further on that a false recollection may set out either from the idea of
some actual past occurrence or from a present product of the
imagination.
It is to be observed, however, that the line of separation between
illusion and hallucination, as thus defined, is a very narrow one. In by
far the largest number of hallucinations it is impossible to prove that
there is no modicum of external agency co-operating in the production
of the effect. It is presumable, indeed, that many, if not all,
hallucinations have such a basis of fact. Thus, the madman who
projects his internal thoughts outwards in the shape of external voices
may, for aught we know, be prompted to do so in part by faint
impressions coming from the ear, the result of those slight stimulations
to which the organ is always exposed, even in profound silence, and
which in his case assume an exaggerated intensity. And even if it is
clearly made out that there are hallucinations in the strict sense, that is
to say, false perceptions which are wholly due to internal causes, it
must be conceded that illusion shades off into hallucination by steps
which it is impossible for science to mark. In many cases it must be left
an open question whether the error is to be classed as an illusion or as a
hallucination.[2]

For these reasons, I think it best not to make the distinction between
illusion and hallucination the leading principle of my classification.
However important psychologically, it does not lend itself to this
purpose. The distinction must be kept in view and illustrated as far as
possible. Accordingly, while in general following popular usage and
employing the term illusion as the generic name, I shall, when
convenient, recognize the narrow and technical sense of the term as
answering to a species co-ordinate with hallucination.
Departing, then, from what might seem the ideally best order of
exposition, I propose, after all, to set out with the simple popular
scheme of faculties already referred to. Even if they are,
psychologically considered, identical operations, perception and
memory are in general sufficiently marked off by a speciality in the
form of the operation. Thus, while memory is the reproduction of
something with a special reference of consciousness to its past
existence, perception is the reproduction of something with a special
reference to its present existence as a part of the presented object. In
other words, though largely representative when viewed as to its origin,
perception is presentative in relation to the object which is supposed to
be immediately present to the mind at the moment.[3] Hence the
convenience of recognizing the popular classification, and of making it
our starting-point in the present case.
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