Metaphysical Realism is a self-contradictory mixture of Naïve Realism and Idealism. Its 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 thought. But he cannot make up his mind at the same time to acknowledge that the mode of existence which thought 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 thought 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 aggregate 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. When the Metaphysical Realist asserts that, beside the ideal relation between the perceived object and the perceiving subject, there must be a real relation between the percept as "thing-in-itself" and the subject as "thing-in-itself" (the so-called individual mind), he is basing his assertion on the false assumption of a real process, imperceptible but analogous to the processes in the world of percepts. Further, when the Metaphysical Realist asserts that we stand in a conscious ideal relation to our world of percepts, but that to the real world we can have only a dynamic (force) relation, he repeats the mistake we have already criticised. We can talk of a dynamic relation only within the world of percepts (in the sphere of the sense of touch), but not outside that world. Let us call the view which we have just characterised, and into which Metaphysical Realism merges when it discards its contradictory elements, Monism, because it combines one-sided Realism and Idealism into a higher unity. For Naïve Realism, the real world is an aggregate of percepts; for Metaphysical Realism, reality belongs not only to percepts but also to imperceptible forces; Monism replaces forces by ideal relations which are supplied by thought. These relations are the laws of nature. A law of nature is nothing but the conceptual expression for the connection of certain percepts.