t 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
or appear to do so (for we must not yet assume that there are such)
after a certain kind of philosophic reflectio