In order to understand the great value and import of Dr. Mierzejewski's investigations, I must remind you that the human brain is composed of two kinds of nerve structure, of an essentially different nature, grey matter and white matter. Examined microscopically, the grey matter is found to be composed of cells, while the white matter consists of fibres; their function also is different, the former being regarded as the generator of nerve force, while the latter simply serves as the medium by which this force is transmitted. As the manifestation of the intellectual powers is supposed to be in some way connected with the development of the grey matter of the cerebral convolutions, one would expect to find in idiots a deficiency of this element of brain tissue.[32] Dr. Mierzejewski maintained that this is by no means the case, and he mentioned an instance of an idiot in whose brain the surface of grey matter was enormous. So it would seem that there is no fixed relation between the amount of grey matter of the brain and intellectual power, for richness of grey substance and abundance of nerve cells may be accompanied by idiocy. Now, as these startling statements of the Russian professor were not made in a hole and corner, but were enunciated in the presence of leading psychologists from all parts of the world, I felt myself justified in telling the materialists that they must be faced, and either answered or admitted as correct; and as my comments upon these experiments were subsequently published in a leading London periodical and widely circulated, I am now justified in assuming that the inferences I then drew from these remarkable experiments cannot be controverted, and that the time has not yet arrived when the broad distinctions between mind and matter are to be obliterated, and man reduced to a mere automaton, a creature of a blind necessity. Without unduly exaggerating the importance of Dr. Mierzejewski's experiments, it must be admitted that very great interest attaches to them at this juncture, when attention is so widely directed to the mysterious connection between matter and mind. Unhappily, instead of solving the question, the Russian professor's researches tend to shroud it in a still deeper mystery, and show that what has been termed the "slippery force of thought--the _vis vivida animæ_"--cannot be weighed in the balance; and they fully justify the eloquent language of a recent writer when he says, "Far more transcendent than all the glories of the universe is the mind of man. Mind is indeed an enigma, the solution of which is apparently beyond the reach of this very mind, itself the problem, the demonstrator, the demonstration, and the demonstrants."