supplemented by the further one that thinking leads us into the part of reality which the percept conceals in itself. Another difficulty in the way of the unprejudiced study of the relation we have here described, between percept and concept as elaborated by thought, may be met with occasionally, when in the field of physics the necessity arises of speaking, not of immediately perceptible elements, but of non-perceptible magnitudes, such as, e.g., lines of electric or magnetic force. It may seem as if the elements of reality of which physicists speak, had no connection either with what is perceptible, or with the concepts which active thinking has elaborated. Yet such a view would depend on self-deception. The main point is that all the results of physical research, except illegitimate hypotheses which ought to be excluded, have been gained through perceiving and conceiving. Entities which are seemingly non-perceptible, are referred by the physicists' sound instinct for knowledge to the field in which actual percepts lie, and they are dealt with in thought by means of the concepts which are commonly applied in this field. The magnitudes in a field of electric or magnetic force are reached, in their essence, by no other cognitive process than the one which connects percept and concept.--An increase or a modification of human senses would yield a different perceptual picture, an enrichment or a modification of human experience. But genuine knowledge could be gained out of this new experience only through the mutual co-operation of concept and percept. The deepening of knowledge depends on the powers of intuition which express themselves in thinking (see page 90). Intuition may, in those experiences in which thinking expresses itself, dive either into deeper or shallower levels of reality. An expansion of the perceptual picture may supply stimuli for, and thus indirectly promote, this diving of intuition. But this diving into the depth, through which we attain reality, ought never to be confused with the contrast between a wider and a narrower perceptual picture, which always contains only half of reality, as that is conditioned by the structure of the knower's organism. Those who do not lose themselves in abstractions will understand how for a knowledge of human nature the fact is relevant, that physics must infer the existence, in the field of percepts, of elements to which no sense is adapted as it is to colour or sound. Human nature, taken concretely, is determined not only by what, in virtue of his physical organisation, man opposes to himself as immediate percept, but also by all else which he excludes from this immediate percept. Just as life needs unconscious sleep alongside of conscious waking experience, so man's experience of himself needs over and above the sphere of his sense-perception another sphere--and a much bigger one--of non-perceptible elements belonging to the same field from which the percepts of the senses come.