All true science that deserves the name is based on a collection of experiences, and consists of conclusions that have been reached by a rational connection of these experiences. "Only in experience is there truth," says Kant. The external world is the object that acts on man's organs of sense, and in the internal sense-centres of the cortex of the brain these impressions are subjectively transformed into presentations. The thought-centres, or association centres, of the cortex (whether or no one distinguishes them from the sense-centres) are the real organs of the mind that unite these presentations into conclusions. The two methods of forming these conclusions--induction and deduction, the formation of arguments and concepts, thought and consciousness--make up together the cerebral function we call reason. These long familiar and fundamental truths, the recognition of which I have described for thirty-eight years as the first condition for solving the riddle of life, are still far from being generally appreciated. On the contrary, we find them combated by the extreme representatives of both tendencies of science. On the one side, the empirical and descriptive school would reduce the whole task to experience, without calling in the aid of philosophy; while philosophic speculation, on the other side, would dispense with experience and endeavor to construct the world by pure thought. Starting from the correct principle that all science originally has its source in experience, the representatives of "experimental science" affirm that their task consists solely in the exact observation of "facts" and the classification and description of them, and that philosophic speculation is nothing more than an idle play of ideas. Hence this one-sided sensualism, as Condillac and Hume especially maintained it, affirmed that the whole action of the mind consists in a manipulation of sense-impressions. This narrow empirical conception spread very widely during the nineteenth century, particularly in the second half, among the rapidly advancing sciences; it was favored by the specialism which grew up in the necessary division of labor. The majority of scientists are still of opinion that their task is confined to the exact observation and description of facts. All that goes beyond this, and especially all far-reaching philosophic conclusions from their accumulated observations, are regarded by them with suspicion. Rudolph Virchow strongly emphasized this narrow empirical tendency ten years ago. In his speech on the foundation of the Berlin University he explained the "transition from the philosophic to the scientific age"; he said that the sole aim of science is "the knowledge of facts, the objective investigation of natural phenomena in detail." The former politician seemed to forget that he had maintained a precisely opposite view forty years before (at Würtzburg), and that his own great achievement, the creation of cellular pathology, was a