7. Are There Limits to Knowledge?

Metaphysical realism is a contradictory mixture of naïve realism and idealism. Its hypothetical forces are imperceptible entities endowed with the qualities of percepts. The metaphysical realist has made up his mind to acknowledge, in addition to the sphere which he is able to know through perception, another sphere for which this means of knowledge fails him and which can be known only by means of thinking. But he cannot make up his mind at the same time to acknowledge that the mode of existence which thinking reveals, namely, the concept (idea), is just as important a factor as the percept. If we are to avoid the contradiction of imperceptible percepts, we must admit that the relationships which thinking establishes between the percepts can have no other mode of existence for us than that of concepts. If we reject the untenable part of metaphysical realism, the world presents itself to us as the sum of percepts and their conceptual (ideal) relationships. Metaphysical realism would then merge into a view of the world which requires the principle of perceivability for percepts and that of conceivability for the relationships between the percepts. This view of the world can admit no third sphere—in addition to the world of percepts and the world of concepts—in which both the so-called “real” and “ideal” principles are simultaneously valid.