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 reflection. And some attempt will be
made to determine roughly how far the process of dissolving these
substantial beliefs of mankind into airy phantasms may venture to go.

For the present, however, these so-called illusions in philosophy will
be ignored. It is plain that illusion exists only in antithesis to real
knowledge. This last must be assumed as something above all question.
And a rough and provisional, though for our purpose sufficiently
accurate, demarcation of the regions of the real and the illusory seems