However divergent are the different views as to the nature and origin of consciousness, they may, nevertheless, on a clear and logical examination, all be reduced to two fundamental theories--the transcendental (or dualistic) and the physiological (or monistic). I have myself always held the latter view, in the light of my evolutionary principles, and it is now shared by a great number of distinguished scientists, though it is by no means generally accepted. The transcendental theory is the older and much more common; it has recently come once more into prominence, principally through Du Bois-Reymond, and it has acquired a great importance in modern discussions of cosmic problems through his famous "Ignorabimus speech." On account of the extreme importance of this fundamental question we must touch briefly on its main features. In the celebrated discourse on "The Limits of Natural Science," which E. du Bois-Reymond gave on August 14, 1872, at the Scientific Congress at Leipzig, he spoke of two "absolute limits" to our possible knowledge of nature which the human mind will never transcend in its most advanced science--_never_, as the oft-quoted termination of the address, "Ignorabimus," emphatically pronounces. The first absolutely insoluble "world-enigma" is the "connection of matter and force," and the distinctive character of these fundamental natural phenomena; we shall go more fully into this "problem of substance" in the twelfth chapter. The second insuperable difficulty of philosophy is given as the problem of consciousness--the question how our mental activity is to be explained by material conditions, especially movements, how "substance [the substance which underlies matter and force] comes, under certain conditions, to feel, to desire, and to think." For brevity, and in order to give a characteristic name to the Leipzig discourse, I have called it the "Ignorabimus speech"; this is the more permissible, as E. du Bois-Reymond himself, with a just pride, eight years afterwards, speaking of the extraordinary consequences of his discourse, said: "Criticism sounded every possible note, from friendly praise to the severest censure, and the word 'Ignorabimus,' which was the culmination of my inquiry, was at once transformed into a kind of scientific shibboleth." It is quite true that loud praise and approbation resounded in the halls of the dualistic and spiritualistic philosophy, and especially in the camp of the "Church militant"; even the spiritists and the host of believers, who thought the immortality of their precious souls was saved by the "Ignorabimus," joined in the chorus. The "severest censure" came at first only from a few scientists and philosophers--from the few who had sufficient scientific knowledge and moral courage to oppose the dogmatism of the all-powerful secretary and dictator of the Berlin Academy of Science.