VII. Are There Limits to Knowledge?

Metaphysical Realism is a heterogeneous mixture of Naive Realism and Idealism. Its hypothetical forces are imperceptible entities endowed with the qualities proper to percepts. The Metaphysical Realist has made up his mind to acknowledge, in addition to the sphere for the existence of which he has an instrument of knowledge in sense-perception, the existence of another sphere for which this instrument fails, 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, viz., the concept (or Idea), has equal rights with percepts. If we are to avoid the contradiction of imperceptible percepts, we must admit that, for us, the relations which thinking traces between percepts can have no other mode of existence than that of concepts. If one rejects the untenable part of Metaphysical Realism, there remains the concept of the world as the sum of percepts and their conceptual (ideal) relations. Metaphysical Realism, then, merges itself in a view of the world according to which the principle of perceptibility holds for percepts, and that of conceivability for the relations between the percepts. This view of the world has no room, in addition to the perceptual and conceptual worlds, for a third sphere, in which both principles, the so-called “real” principle and the “ideal” principle, are simultaneously valid.