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.
 When the metaphysical realist asserts that, besides the ideal relationship between the percept of the object and the percept of the subject, there must also exist a real relationship between the "thing-in-itself" of the percept and the "thing-in-itself" of the perceptible subject (that is, of the so-called individual spirit), he is basing his assertion on the false assumption of a real process, analogous to the processes in the sense world but imperceptible. Further, when the metaphysical realist asserts that we enter into a conscious ideal relationship to our world of percepts, but that to the real world we can have only a dynamic (force) relationship, he repeats the mistake we have already criticized. One can talk of a dynamic relationship 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 characterized above, into which metaphysical realism merges when it discards its contradictory elements, monism, because it combines one-sided realism with idealism into a higher unity.
 For naïve realism, the real world is an aggregate of perceived objects (percepts); for metaphysical realism, not only percepts but also imperceptible forces are real; monism replaces forces by ideal connections which are gained through thinking. The laws of nature are just such connections. A law of nature is in fact nothing but the conceptual expression of the connection between certain percepts.