Naive realism, with its fundamental principle of the reality of all perceived things, is contradicted by experience, which teaches us that the content of percepts is of a transitory nature. The tulip I see is real today; in a year it will have vanished into nothingness. What persists is the species tulip. For the naïve realist, however, this species is "only" an idea, not a reality. Thus this theory of the world find itself in the position of seeing its realities arise and perish, while what it regards as unreal, in contrast with the real, persists. Hence naïve realism is compelled to acknowledge, in addition to percepts, the existence of something ideal. It must admit entities which cannot be perceived by the senses. In doing so, it justifies itself by conceiving their existence as being analogous to that of sense-perceptible objects. Just such hypothetical realities are the invisible forces by means of which the sense-perceptible objects act on one another. Another such thing is heredity, which works on beyond the individual and is the reason why a new being which develops from the individual is similar to it, thereby serving to maintain the species. Such a thing again is the life-principle permeating the organic body, the soul for which the naïve mind always finds a concept formed in analogy with sense realities, and finally the naïve man's Divine Being. This Divine Being is thought of as acting in a manner exactly corresponding to the way in which man himself is seen to act; that is, anthropomorphically.
 Modern physics traces sensations back to processes of the smallest particles of bodies and of an infinitely fine substance, called ether, or to other such things. For example, what we experience as warmth is, within the space occupied by the warmth-giving body, the movement of its parts. Here again something imperceptible is conceived in analogy with what is perceptible. In this sense, the perceptual analogue to the concept "body" would be, shall we say, the interior of a totally enclosed space, in which elastic spheres are moving in all directions, impinging one on another, bouncing on and off the walls, and so on.