Every kind of existence that is assumed outside the realm of percept and concept must be relegated to the sphere of unjustified hypotheses. To this category belongs the "thing-in-itself". It is quite natural that a dualistic thinker should be unable to find the connection between the world principle which he hypothetically assumes and the things given in experience. A content for the hypothetical world principle can be arrived at only by borrowing it from the world of experience and then shutting one's eyes to the fact of the borrowing. Otherwise it remains an empty concept, a non-concept which has nothing but the form of a concept. Here the dualistic thinker usually asserts that the content of this concept is inaccessible to our cognition; we can know only that such a content exists, but not what it is that exists. In both cases it is impossible to overcome dualism. Even though one were to import a few abstract elements from the world of experience into the concept of the thing-in-itself, it would still remain impossible to derive the rich concrete life of experience from these few qualities which are, after all, themselves taken from perception. DuBois-Reymond considers that the imperceptible atoms of matter produce sensation and feeling by means of their position and motion, and then comes to the conclusion that we can never find a satisfactory explanation of how matter and motion produce sensation and feeling, for "it is absolutely and for ever incomprehensible that it should be other than indifferent to a number of atoms of carbon, hydrogen, nitrogen, and so on, how they lie and move, how they lay and moved, or how they will lie and will move. It is impossible to see how consciousness could come into existence through their interaction." This conclusion is characteristic of this whole trend of thought. Position and motion are abstracted from the rich world of percepts, They are then transferred to the notional world of atoms. And then astonishment arises that real life cannot be evolved out of this self-made principle borrowed from the world of percepts.
 That the dualist can reach no explanation of the world, working as he does with a completely empty concept of the "in-itself" of a thing, follows at once from the very definition of his principle given above.
 In every case the dualist finds himself compelled to set impassable barriers to our capacity for cognition. The follower of a monistic world conception knows that everything he needs for the explanation of any given phenomenon in the world must lie within this world itself. What prevents him from reaching it can be only accidental limitations in space and time, or defects of his organization, that is, not of human organization in general, but only of his own particular one.