The self-contained being of ideas is not thought of by the naïve mind as real in the same sense. An object conceived “merely in idea” is regarded as a chimera until sense-perception can furnish proof of its reality. In short, the naïve man demands, in addition to the ideal evidence of his thinking, the real evidence of his senses. In this need of the naïve man lies the ground for the origin of the belief in revelation. The God whom we apprehend by thought remains always merely our idea of God. The naïve consciousness demands that God should manifest Himself in ways accessible to the senses. God must appear in the flesh, and must attest his Godhead to our senses by the changing of water into wine.
Even knowledge itself is conceived by the naïve mind as a process analogous to sense-perception. Things, it is thought, make an impression on the mind, or send out copies of themselves which enter through our senses, etc.
What the naïve man can perceive with his senses he regards as real, and what he cannot perceive (God, soul, knowledge, etc.) he regards as analogous to what he can perceive.
On the basis of Naïve Realism, science can consist only in an exact description of the content of perception. Concepts are only means to this end. They exist to provide ideal counterparts of percepts. With the things themselves they have nothing to do. For the Naïve Realist only the individual tulips, which we can see, are real. The universal idea of tulip is to him an abstraction, the unreal thought-picture which the mind constructs for itself out of the characteristics common to all tulips.
Naïve Realism, with its fundamental principle of the reality of all percepts, contradicts experience, which teaches us that the content of percepts is of a transitory nature. The tulip I see is real to-day; in a year it will have vanished into nothingness. What persists is the species “tulip.” This species is, however, for the Naïve Realist merely an idea, not a reality. Thus this theory of the world finds itself in the paradoxical position of seeing its realities arise and perish, while that which, by contrast with its realities, it regards as unreal endures. Hence Naïve Realism is compelled to acknowledge the existence of something ideal by the side of percepts. It must include within itself entities which cannot be perceived by the senses. In admitting them, it escapes contradicting itself by conceiving their existence as analogous to that of objects of sense. Such hypothetical realities are the invisible forces by means of which the objects of sense-perception act on one another. Another such reality is heredity, the effects of which survive the individual, and which is the reason why from the individual a new being develops which is similar to it, and by means of which the species is maintained. The soul, the life-principle permeating the organic body, is another such reality which the naïve mind is always found conceiving in analogy to realities of sense-perception. And, lastly, the Divine Being, as conceived by the naïve mind, is such a hypothetical entity. The Deity is thought of as acting in a manner exactly corresponding to that which we can perceive in man himself, i.e., the Deity is conceived anthropomorphically.
Modern Physics traces sensations back to the movements of the smallest particles of bodies and of an infinitely fine substance, called ether. What we experience, e.g., as warmth is a movement of the parts of a body which causes the warmth in the space occupied by that body. Here again something imperceptible is conceived on the analogy of what is perceptible. Thus, in terms of perception, the analogon to the concept “body” is, say, the interior of a room, shut in on all sides, in which elastic balls are moving in all directions, impinging one on another, bouncing on and off the walls, etc.
Without such assumptions the world of the Naïve Realist would collapse into a disconnected chaos of percepts, without mutual relations, and having no unity within itself. It is clear, however, that Naïve Realism can make these assumptions only by contradicting itself. 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 of which perceptible things are the bearers are, in fact, illegitimate hypotheses from the standpoint of Naïve Realism. But because Naïve Realism knows no other realities, it invests its hypothetical forces with perceptual content. It thus transfers a form of existence (the existence of percepts) to a sphere where the only means of making any assertion concerning such existence, viz., sense-perception, is lacking.
This self-contradictory theory leads to Metaphysical Realism. The latter constructs, beside the perceptible reality, an imperceptible one which it conceives on the analogy of the former. Metaphysical Realism is, therefore, of necessity Dualistic.
Wherever the Metaphysical Realist observes a relation between perceptible things (mutual approach through movement, the entrance of an object into consciousness, etc.), there he posits a reality. However, the relation of which he becomes aware cannot be perceived but only expressed by means of thought. The ideal relation is thereupon arbitrarily assimilated to something perceptible. Thus, according to this theory, the world is composed of the objects of perception which are in ceaseless flux, arising and disappearing, and of imperceptible forces by which the perceptible objects are produced, and which are permanent.
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.