I. Haeckel and His Opponents

Now, however, the dualistic world-conception is disproved by nothing more effectively than by the consideration of the human mind. When I want to explain an external occurrence—for instance, the motion of an elastic ball which has been struck by another, I cannot stop short at the mere observation, but must seek the law which determines the direction of motion and velocity of the one ball from the direction and velocity of the other. Mere observation cannot furnish me with such a law, but only the linking together in thought of what happens. Man, therefore, draws from his mind the means of explaining that which presents itself to him through observation. He must pass beyond the mere observation, if he wants to comprehend it. Observation and thought are the two sources of our knowledge about things; and that holds good for all things and happenings, except only for the thinking consciousness itself. To that we cannot add by any explanation anything that does not lie already in the observation itself. It yields us the laws for all other things; it yields us at the same time its own laws also. If we want to demonstrate the correctness of a natural law, we accomplish this by distinguishing, arranging observations and perceptions, and drawing conclusions—that is, we form conceptions and ideas about the experiences in question with the help of thinking. As to the correctness of the thinking, thought itself alone decides. It is thus thought which, in regard to all that happens in the world, carries us beyond mere observation, though it does not carry us beyond itself.