"I am convinced," he exclaims, at the end of his preface, "that I shall never find myself compelled to deny the thesis of _the unity_ of human nature." Unhappily, this "conviction" proved to be a grave error. Twenty-eight years afterwards Virchow represented the diametrically opposite view; it is to be found in the famous speech on "The Liberty of Science in Modern States," which he delivered at the Scientific Congress at Munich in 1877, and which contains attacks that I have repelled in my _Free Science and Free Teaching_ (1878). In Emil du Bois-Reymond we find similar contradictions with regard to the most important and fundamental theses of philosophy. The more completely the distinguished orator of the Berlin Academy had defended the main principles of the monistic philosophy, the more he had contributed to the refutation of vitalism and the transcendental view of life, so much the louder was the triumphant cry of our opponents when in 1872, in his famous _Ignorabimus-Speech_, he spoke of consciousness as an insoluble problem, and opposed it to the other functions of the brain as a supernatural phenomenon. I return to the point in the tenth chapter. The peculiar character of many of the psychic phenomena, especially of consciousness, necessitates certain modifications of our ordinary scientific methods. We have, for instance, to associate with the customary _objective_, external observation, the _introspective_ method, the _subjective_, internal observation which scrutinizes our own personality in the mirror of consciousness. The majority of psychologists have started from this "certainty of the ego": "_Cogito ergo sum_," as Descartes said--I think, therefore I am. Let us first cast a glance at this way of inquiry, and then deal with the second, complementary, method.