[22] Without such assumptions the world would fall apart for the naïve realist into an incoherent aggregate of percepts without mutual relationships and with no tendency to unite. It is clear, however, that naïve realism can make these assumptions only by an inconsistency. If it would remain true to its fundamental principle that only what is perceived is real, then it ought not to assume a reality where it perceives nothing. The imperceptible forces which proceed from the perceptible things are in fact unjustified hypotheses from the standpoint of naïve realism. And because naïve realism knows no other realities, it invests its hypothetical forces with perceptual content. It thus ascribes a form of existence (perceptible existence) to a sphere where the only means of making any assertion about such existence, namely, sense perception, is lacking.

[23] This self-contradictory theory leads to metaphysical realism. This constructs, in addition to the perceptible reality, an imperceptible reality which it conceives on the analogy of the perceptible one. Therefore metaphysical realism is of necessity dualistic.

[24] Wherever the metaphysical realist observes a relationship between perceptible things (such as when two things move towards each other, or when something objective enters consciousness), there he sees a reality. However, the relationship which he notices can only be expressed by means of thinking; it cannot be perceived. The purely ideal relationship is then arbitrarily made into something similar to a perceptible one. Thus, according to this theory, the real world is composed of the objects of perception which are in ceaseless flux, arising and disappearing, and of imperceptible forces which produce the objects of perception, and are the things that endure.