THE theory of perception is perhaps the most important as well as the most difficult problem of psychology. The interpretation of the higher processes of mind rests upon it and it underlies the body of our general philosophy. The great philosophies of the world take their rise from initial differences in the method of construing perception. Leaving the general problems of the theory of knowledge to metaphysics, we have to do only with the process of perception, considered as an operation of mind in attaining knowledge of the external world. That is, we have to answer the simple question, "How do we arrive at the knowledge of individual objects localized in space and time?" In view of the terms of this question and of the analysis which follows, we may define perception in a general way.
Perception is the apperceptive or synthetic process of mind whereby the data of sensation take on the forms of representation in space and time: or, considered more with reference to things external to us, it is the process of the construction of our representation of the external world.
A little reflection leads to the conclusion that our perception of the external world is a matter of mental construction. All advance into the region of mind must be through mental states. The characteristic of mind is consciousness, and nothing can enter the domain of mind except through the mediation of consciousness. This is seen in the fact that our images play in consciousness in such a way as