The theory of perception is fundamental both in normal and abnormal psychology. All mental activities are intimately related with the process of perception. Our wills, our thoughts and our feelings relate to our experience of the outer world of things. Biologically regarded, the percept is of the most vital importance, inasmuch as it forms the medium between the individual and the outer environment. Psychologically, the percept reflects the external world and mirrors the conditions of life to which the given organism has to adjust itself. In fact, the percept may be regarded as the coin possessing the value of the external environment. In this respect we cannot help agreeing with Baldwin's statement: "The theory of perception is perhaps the most important as well as the most difficult problem in 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."
In abnormal psychology the theory of perception is of the utmost importance, both from a theoretical and practical standpoint. Illusions, hallucinations, dream states, subconscious states, many states of dissociation depend for their explanation on the analysis of the process of perception. I have developed a theory of perception which may be characterized as the doctrine of primary and secondary sensory elements. This doctrine is based on a close analysis of the normal process of perception and is substantiated by observations and experiments of abnormal mental life.
Before however we state our view of perception it may be well to make a review of what the principal psychological authorities teach on the subject.
James Mill in discussing perception tells us: "The colors upon a body are different, according to its figure, its shape, and its size. But the sensations of color and the sensations of extension, of figure, of distance have been so often united, felt in conjunction that the sensations of the color are never experienced without raising the ideas of the extension; the figure, the distance in such intimate union with it, that they not only cannot be separated, but are actually supposed to be seen (italics are mine). The sight, as it is called of figure, or distance, appearing, as it does a simple sensation, is in reality a complex state of consciousness, a sequence in which the antecedent, a sensation of color, and the consequent a number of ideas are so closely combined by association that they appear not one idea, but one sensation."
Sully defines perception as a mental act that ‘supplements a sense impression by an accompaniment or escort of revived sensations, the whole aggregate of actual and revived sensations being solidified or integrated into the form of a percept.' The revived sensations are equivalent to James Mill's associated ideas and images.
We shall point out later the confusion which generally prevails among psychologists and psychiatrists, when they talk indiscriminately of revived sensations and ideas regarding the two as identical.
Höffding describes the process of perception as the fusing of a reproduction and an actual sensation. The percept is thus conceived as compounded out of a representation and a sensation."
Taine tells us that: "Images associated with the sensations of the different senses, especially with those of sight and touch constitute acquired perceptions."
Wundt regards the percept as a psychical compound of ideas or of revived sensations or images. In that respect his analysis differs but little from that of other psychologists who regard the ideas, images, and revived sensations as identical elements going to form the associated whole or psychic compound, the percept.
Külpe speaks of ‘centrally excited sensations' regarding them as the ideas and the images of the psychologists and psychiatrists, and tells us that he avoids the use of 'ideas.' As far as perception is concerned he closely follows his master, Wundt, and talks of psychic compounds, of sensations and centrally excited sensations which really are identical with the old ideas and images.