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.

All knowledge which has any appearance of being directly reached,
immediate, or self-evident, that is to say, of not being inferred from
other knowledge, may be divided into four principal varieties: Internal
Perception or Introspection of the mind's own feelings;