ion? When trying to fix the definition of illusion in
general, I practically disposed of this question. Nevertheless, as the
point appears to me to be of some importance, I shall reproduce and
expand one or two of the considerations then brought forward.

It is said by certain, philosophers that perception, as a whole, is an
illusion, inasmuch as it involves the fiction of a real thing
independent of mind, yet somehow present to it in the act of
sense-perception. But this is a question for philosophy, not for
science. Science, including psychology, assumes that in perception there
is something real, without inquiring what it may consist of, or what its
meaning may be. And though in the foregoing analysis of perception,
viewed as a complex mental phenomenon or psychical process, I have
argued that a percept gets its concrete filling up out of elements of
conscious experience or sensations, I have been careful not to contend
that the particular elements of feeling thus represented are the
_object_ of perception or the thing perceived. It may be that what we
mean by a single object with its assemblage of qualities is much more
than any number of such sensations; and it must be confessed that, on
the face of it, it seems to be much more. And however this be, the
question, What is meant by object; and is the common persuasion of the
existence of such an entity in the act of perception accurate or
illusory? must be handed over to philosophy.

While in the following examination of sense-illusions we put out of
sight what certain philosophers say about the illusoriness of perception
as a whole, we shall also do well to leave out of account what physical
science is sometimes supposed to tell us respecting a constant element
of illusion in perception. The physicist, by reducing all external
changes to "modes of motion," appears to leave no room in his
world-mechanism for the secondary qualities of bodies, such as light
and heat, as popularly conceived. Yet, while allowing this, I think we
may still regard the attribution of qualities like colour to objects as
in the main correct and answering to a real fact. When a person says an
object is red, he is understood by everybody as affirming something
which is true or false, something therefore which either involves an
external fact or is illusory. It would involve an external fact whenever
the particular sensation which he receives is the result of a physical
action (other vibration