The means and methods we have chosen for attaining the solution of the great enigma do not differ, on the whole, from those of all purely scientific investigation--firstly, experience; secondly, inference. Scientific experience comes to us by observation and experiment, which involve the activity of our sense-organs in the first place, and, secondly, of the inner sense-centres in the cortex of the brain. The microscopic elementary organs of the former are the sense-cells; of the latter, groups of ganglionic cells. The experiences which we derive from the outer world by these invaluable instruments of our mental life are then moulded into ideas by other parts of the brain, and these, in their turn, are united in a chain of reasoning by association. The construction of this chain may take place in two different ways, which are, in my opinion, equally valuable and indispensable: _induction_ and _deduction_. The higher cerebral operations, the construction of complicated chains of reasoning, abstraction, the formation of concepts, the completion of the perceptive faculty by the plastic faculty of the imagination--in a word, consciousness, thought, and speculation--are functions of the ganglionic cells of the cortex of the brain, just like the preceding simpler mental functions. We unite them all in the supreme concept of _reason_.[6] By reason only can we attain to a correct knowledge of the world and a solution of its great problems. Reason is man's highest gift, the only prerogative that essentially distinguishes him from the lower animals. Nevertheless, it has only reached this high position by the progress of culture and education, by the development of knowledge. The uneducated man and the savage are just as little (or just as much) "rational" as our nearest relatives among the mammals (apes, dogs, elephants, etc.). Yet the opinion still obtains in many quarters that, besides our godlike reason, we have two further (and even surer!) methods of receiving knowledge--emotion and revelation. We must at once dispose of this dangerous error. Emotion has nothing whatever to do with the attainment of truth. That which we prize under the name of "emotion" is an elaborate activity of the brain, which consists of feelings of like and dislike, motions of assent and dissent, impulses of desire and aversion. It may be influenced by the most diverse activities of the organism, by the cravings of the senses and the muscles, the stomach, the sexual organs, etc. The interests of truth are far from promoted by these conditions and vacillations of emotion; on the contrary, such circumstances often disturb that reason which alone is adapted to the pursuit of truth, and frequently mar its perceptive power. No cosmic problem is solved, or even advanced, by the cerebral function we call emotion. And the same must be said of the so-called "revelation," and of the "truths of faith" which it is supposed to communicate; they are