s of a certain order), which would produce a like
sensation in anybody else in the same situation and endowed with the
normal retinal sensibility. On the other hand, an illusory attribution
of colour would imply that there is no corresponding physical agency at
work in the case, but that the sensation is connected with exceptional
individual conditions, as, for example, altered retinal sensibility.

We are now, perhaps, in a position to frame a rough definition of an
illusion of perception as popularly understood. A large number of such
phenomena may be described as consisting in the formation of percepts or
quasi-percepts in the minds of individuals under external circumstances
which would not give rise to similar percepts in the case of other
people.

A little consideration, however, will show that this is not an adequate
definition of what is ordinarily understood by an illusion of sense.
There are special circumstances which are fitted to excite a momentary
illusion in all minds. The optical illusions due to the reflection and
refraction of light are not peculiar to the individual, but arise in all
minds under precisely similar external conditions.

It is plain that the illusoriness of a perception is in these cases
determined in relation to the sense-impressions of other moments and
situations, or to what are presumably better percepts than the present
one. Sometimes this involves an appeal from one sense to another. Thus,
there is the process of verification of sight by touch, for example, in
the case of optical images, a mode of perception which, as we have seen,
gives a more direct cognition of external quality. Conversely, there may
occasionally be a reference from touch to sight, when it is a question
of discriminating two points lying very close to one another. Finally,
the same sense may correct itself, as when the illusion of the
stereoscope is corrected by afterwards looking at the two separate
pictures.

We may thus roughly define an illusion of perception as consisting in
the formation of a quasi-percept which is peculiar to an individual, or
which is contradicted by another and presumably more accurate percept.
Or, if we take the meaning of the word common to include both the
universal as contrasted with the individual experience, and the
permanent, constant, or average, as distinguished from the momentary and
variable percept, we may still briefly describe an illusion of
perception as a deviation f