based entirely on a deception, consciously or unconsciously, as we shall see in the sixteenth chapter. We must welcome as one of the most fortunate steps in the direction of a solution of the great cosmic problems the fact that of recent years there is a growing tendency to recognize the two paths which alone lead thereto--_experience_ and _thought_, or _speculation_--to be of equal value, and mutually complementary. Philosophers have come to see that pure speculation--such, for instance, as Plato and Hegel employed for the construction of their _idealist_ systems--does not lead to knowledge of reality. On the other hand, scientists have been convinced that mere experience--such as Bacon and Mill, for example, made the basis of their _realist_ systems--is insufficient of itself for a complete philosophy. For these two great paths of knowledge, sense-experience and rational thought, are two distinct cerebral functions; the one is elaborated by the sense-organs and the inner sense-centres, the other by the thought-centres, the great "centres of association in the cortex of the brain," which lie between the sense-centres. (Cf. cc. vii. and x.) True knowledge is only acquired by combining the activity of the two. Nevertheless, there are still many philosophers who would construct the world out of their own inner consciousness, and who reject our empirical science precisely because they have no knowledge of the real world. On the other hand, there are many scientists who still contend that the sole object of science is "the knowledge of facts, the objective investigation of isolated phenomena"; that "the age of philosophy" is past, and science has taken its place.[7] This one-sided over-estimation of experience is as dangerous an error as the converse exaggeration of the value of speculation. Both channels of knowledge are mutually indispensable. The greatest triumphs of modern science--the cellular theory, the dynamic theory of heat, the theory of evolution, and the law of substance--are _philosophic achievements_; not, however, the fruit of pure speculation, but of an antecedent experience of the widest and most searching character. At the commencement of the nineteenth century the great idealistic poet, Schiller, gave his counsel to both groups of combatants, the philosophers and the scientists: "Does strife divide your efforts--no union bless your toil? Will truth e'er be delivered if ye your forces rend?" Since then the situation has, happily, been profoundly modified; while both schools, in their different paths, have pressed onward towards the same high goal, they have recognized their common aspiration, and they draw nearer to a knowledge of the truth in mutual covenant. At the end of the nineteenth century we have returned to that monistic attitude which our greatest realistic poet, Goethe, had recognized from its very commencement to be alone correct and fruitful.[8]