This has all been completely changed in the course of the last half-century. Modern physiology has already to a great extent demonstrated the localisation of the various activities of mind, and their connection with definite parts of the brain; psychiatry has shown that those psychical processes are disturbed or destroyed if these parts of the brain become diseased or degenerate. Histology has revealed to us the extremely complicated structure and arrangement of the ganglion-cells. But, for the settlement of this momentous question, the discoveries of the last ten years with regard to the more minute occurrences in the process of fertilisation are of decisive importance. We now know that this process essentially consists simply in the copulation or fusion of two microscopical cells, the female egg-cell and the male sperm-cell. The fusion of the nuclei of these two sexual cells indicates with the utmost precision the exact moment at which the new human individual arises. The newly-formed parent-cell, or fertilised egg-cell, contains potentially, in their rudiments, all the bodily and mental characteristics which the child inherits from both parents. It is clearly against reason to assume an eternal and unending life for an individual phenomenon whose beginning in time we can determine to a hair's breadth, by direct observation. Judging of human spiritual life from a rational point of view, we can as little think of our individual soul as separated from our brain, as we can conceive the voluntary motion of our arm apart from the contraction of its muscles, or the circulation of our blood apart from the action of the heart. Against this strictly physiological conception, as against our whole monistic view of the relations of energy and matter, of soul and substance, the reproach of "materialism" continues to be raised. I have repeatedly before now pointed out that this is an ambiguous party word which conveys absolutely nothing; its apparent opposite, "spiritualism," could quite easily be substituted for it. Every critical thinker, who is familiar with the history of philosophy, knows that, as systems change, such words assume the most varied meanings, In addition to this, the word "materialism" has the disadvantage of being liable to continual confusion between its theoretical and practical meanings, which two are totally distinct. Our conception of Monism, or the unity-philosophy, on the contrary, is clear and unambiguous; for it an immaterial living spirit is just as unthinkable as a dead, spiritless material; the two are inseparably combined in every atom. The opposed conception of dualism (or even pluralism in other anti-monistic systems) regards spirit and material, energy and matter, as two essentially different substances; but not a single empirical proof can be adduced to show that either of these can exist or become perceptible to us by itself alone.