7. Are There Limits to Knowledge?

Another difficulty in the way of the unprejudiced observation of the relationship between the percept and the concept wrought by thinking, as here described, arises when, for example, in the field of experimental physics it becomes necessary to speak not of immediately perceptible elements, but of non-perceptible quantities as in the case of 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 wrought. Yet such a view would be based on self-deception. The main point is that all the results of physical research, apart from unjustifiable hypotheses which ought to be excluded, have been obtained through percept and concept. Elements which are seemingly non-perceptible are placed by the physicist's sound instinct for knowledge into the field where percepts lie, and they are thought of in terms of concepts commonly used in this field. The strengths of electric or magnetic fields and such like are arrived at, in the very nature of things, by no other process of knowledge than the one which occurs between percept and concept.