I have not yet explained in any formal manner what the nature of that insight is which constitutes what I have named the Principle of Vision; although doubtless the reader has gathered its meaning from the remarks already made. For the sake of future applications of the principle to the various questions of philosophical criticism which must arise in the course of this inquiry, it may be needful here to explain (as I have already explained elsewhere) how the chief intellectual operations—Perception, Inference, Reasoning, and Imagination—may be viewed as so many forms of mental vision.
Perception, as distinguished from Sensation, is the presentation before Consciousness of the details which once were present in conjunction with the object at this moment affecting Sense. These details are inferred to be still in conjunction with the object, although not revealed to Sense. Thus when an apple is perceived by me, who merely see it, all that Sense reports is of a certain coloured surface: the roundness, the firmness, the fragrance, and the taste of the apple are not present to Sense, but are made present to Consciousness by the act of Perception. The eye sees a certain coloured surface; the mind sees at the same instant many other co-existent but unapparent facts—it reinstates in their due order these unapparent facts. Were it not for this mental vision supplying the deficiencies of ocular vision, the coloured surface would be an enigma. But the suggestion of Sense rapidly recalls the experiences previously associated with the object. The apparent facts disclose the facts that are unapparent.