of his brain are destroyed. The physiologist may show from the monkey down to the pigeon, to the frog, to the ant, to the worm, how the behavior of animals is changed as soon as certain groups of nervous elements are extirpated. It is the mental emotional character of the pigeon which is changed when the physiologist cuts off parts of his brain. In short, stimulation and destruction demonstrate, by experiments which supplement each other, that mental functions correspond to brain functions.

There is thus no lack of demonstration from all quarters that mental facts and brain processes belong together; and yet, however much we may cumulate such popular and scientific observations, they would never by themselves admit of the sweeping generalization that there cannot be any mental state which is not accompanied by a process in the central nervous system. Someone might say, to be sure, the perceptions and memory images, the volitions and instincts and impulses, have their physiological basis, but there remain after all acts of attention, or decisions, or subtle feelings, or flights of imagination, which are independent of any brain action. Here, indeed, observation cannot settle such a general principle. Its real hold lies in the fact with which we started: there is no causal connection in the mental states as such. If we want to understand mental facts as such in a chain, of causal events, we have first to conceive them as parallel to physical events. The principle of psychophysical parallelism, that is, the principle that every psychical process accompanies a physiological change is thus not a mere result of observation. It is simply a postulate. Every science begins with postulates and only that which fulfills such postulates has the dignity of truth in the midst of that scientific realm. The astronomer cannot find by observation that there is no star the movements of which are not the effects of foregoing causes. He knows it beforehand, he demands it, he does not recognize any movement as understood until he has found the causes, he presupposes that such causes exist, that no star moves simply by a magic power, and that nowhere in the astronomical universe is the chain of causality broken. He postulates it, and where he does not discover the causes, he is sure that he has not solved the real problem.

In the same way the psychologist who aims towards explanation of mental facts must postulate that there cannot be any mental state which is not an accompaniment of a physical brain process, and is as such connected through physical means with the preceding and the following events in the psychophysical system. Only when such a general framework of theory is built up by a logical postulate, is the way open to make use of all those observations of the laboratory and of the clinic, of the zoölogist and of the anatomist. It is the theory which has to give the right setting to those scattered observations. However far we may be from being able to point to the special brain process which lies at the bottom of the higher mental state, we know beforehand that there is no shadow of an idea, no fringe of a feeling, no suggestion of a desire which does not correspond to definite processes in the brain. The details may and must be material for diverging theories, but the conflict of such hypothetical opinions has nothing to do with the certainty of the underlying conviction that if we knew the whole truth, we should recognize every single mental happening as parallel to physical processes in the nervous system. To explain mental facts means to think them as parallel to the brain processes which have their own causal connections in the physical world.