Towards the end, however, the author of the "Ignorabimus speech" briefly alluded to the question whether these two great "world-enigmas," the general problem of substance and the special problem of consciousness, are not two aspects of one and the same problem. "This idea," he said, "is certainly the simplest, and preferable to the one which makes the world doubly incomprehensible. Such, however, is the nature of things that even here we can obtain no clear knowledge, and it is useless to speak further of the question." The latter sentiment I have always stoutly contested, and have endeavored to prove that the two great questions are not two distinct problems. "The neurological problem of consciousness is but a particular aspect of the all-pervading cosmological problem of substance." The peculiar phenomenon of consciousness is not, as Du Bois-Reymond and the dualistic school would have us believe, a completely "transcendental" problem; it is, as I showed thirty-three years ago, a _physiological_ problem, and, as such, must be reduced to the phenomena of physics and chemistry. I subsequently gave it the more definite title of a _neurological_ problem, as I share the view that true consciousness (thought and reason) is only present in those higher animals which have a centralized nervous system and organs of sense of a certain degree of development. Those conditions are certainly found in the higher vertebrates, especially in the placental mammals, the class from which man has sprung. The consciousness of the highest apes, dogs, elephants, etc., differs from that of man in degree only, not in kind, and the graduated interval between the consciousness of these "rational" placentals and that of the lowest races of men (the Veddahs, etc.) is less than the corresponding interval between these uncivilized races and the highest specimens of thoughtful humanity (Spinoza, Goethe, Lamarck, Darwin, etc.). Consciousness is but a part of the higher activity of the soul, and as such it is dependent on the normal structure of the corresponding psychic organ, the brain. Physiological observation and experiment determined twenty years ago that the particular portion of the mammal-brain which we call the _seat_ (preferably the _organ_) of consciousness is a part of the cerebrum, an area in the late-developed gray bed, or cortex, which is evolved out of the convex dorsal portion of the primary cerebral vesicle, the "fore-brain." Now, the morphological proof of this physiological thesis has been successfully given by the remarkable progress of the microscopic anatomy of the brain, which we owe to the perfect methods of research of modern science (Kölliker, Flechsig, Golgi, Edinger, Weigert, and others).