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 vibrations 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.