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.