With this caution I return to the field of positive knowledge. The brain, spinal marrow, and nerves consist of two substances: one white, which constitutes the great mass consisting of tubes or fibres; the other grey, which is an aggregation of minute cells, so minute that it has been computed that there are several millions of them in a space no larger than a sixpence. The bulk of this grey nerve-tissue is found in the higher animals, and especially in man, in the outside rind which covers the brain, and its amount is greatly increased by the convolutions of that organ giving a greater extent of covering surface. In fact the convolutions of the average human brain give as much grey matter in a head of average size, as would be given by a head of four times the size if the brain were a plane surface. The extent of the convolutions is, therefore, a sure sign of the extent of intellect. They are more numerous and deeper in the European than in the negro; in the negro than in the chimpanzee; in the anthropoid ape than in the monkey or lemur. This grey nerve-tissue is the organ by which impressions from without are turned into perceptions, volitions, and evolutions of nerve force. The white matter is simply the medium of transmission, or we may say the telegraph wires by which the impressions are conveyed to the head office and the answers sent. The cell-tissue of the grey matter is thus emphatically the organ of the mind. In fact, if it did not sound too materialistic, we might call thought a secretion of the grey matter, only in saying so we must bear in mind that it is only a mode of expressing the fact that the two invariably go together; and that if we say with the German philosopher ‘Ohne Phosphor kein Gedank,’ it does not mean that thought and phosphorus are identical, but simply that the condition on which thought depends is that of the existence of a material organ of which phosphorus is an ingredient.