n. 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
to coincide with the line which common sense draws between what all
normal men agree in holding and what the individual holds, whether
temporarily or permanently, in contradiction to this. For our present
purpose the real is that which is true for all. Thus, though physical
science may tell us that there is nothing corresponding to our
sensations of colour in the world of matter and motion which it
conceives as surrounding us; yet, inasmuch as to all men endowed with
the normal colour-sense the same material objects appear to have the
same colour, we may speak of any such perception as practically true,
marking it off from those plainly illusory perceptions which are due to
some subjective cause, as, for example, fatigue of the retina.

To sum up: in treating of illusions we shall assume, what science as
distinguished from philosophy is bound to assume, namely, that human
experience is consistent; that men's perceptions and beliefs fall into a
consensus. From this point of view illusion is seen to arise through
some exceptional feature in the situation or condition of the
individual, which, for the time, breaks the chain of intellectual
solidarity which under ordinary circumstances binds the single member to
the collective body. Whether the common experience which men thus obtain
is rightly interpreted is a question which does not concern us here. For
our present purpose, which is the determination and explanation of
illusion as popularly understood, it is sufficient that there is this
general consensus of belief, and this may provisionally be regarded as
at least practically true.

CHAPTER II.

THE CLASSIFICATION OF ILLUSIONS.

If illusion is the simulation of immediate knowledge, the most obvious
mode of classifying illusions would appear to be according to the
variety of the knowledge which they simulate.

Now, the popular psychology that floats about in the ordinary forms of
language has long sin