Not only are there these formal resemblances, there is a real connection, according to Professor Höffding, as is shown by the fact that with the evolution of the nervous system go higher and higher forms of consciousness, that irritation on the surface of an organism must be communicated to the brain that conscious sensations may arise, and that when arterial blood fails to reach the brain unconsciousness supervenes. What hypothesis do these facts conduct to us? All of them must be born in mind that any special hypothesis may be legitimated. There are only four possibilities: (1) Either consciousness and the brain, mind and body, act upon one another as two separate things or substances; (2) or the mind is only a form or product of the body; (3) or the body is only a form or product of one or more mental substances; (4) or mind and body, consciousness and the brain, grow and develop as different manifestations of one and the same substance. It must be admitted that the author at this point somewhat deserts the empirical standpoint to which he declared at the outset that he should keep. The _facts_ of correspondence or parallelism are all that come within the realm of experience; their _explanation_ must be more or less a matter of inference and theoretical construction and involves a departure in the direction of metaphysics. Professor Höffding is aware of this and says that these hypotheses belong to the border-land between positive science and metaphysics. Moreover, he confesses that any conclusion he may reach will have only a provisional value and may need revision, before it can serve as a final part of a philosophical system. He will, however, follow as closely as possible the leadings of experience, as indeed he says we should do in all metaphysical speculation.
In considering the first hypothesis, (namely, that mind and body act on one another as two things,) Professor Höffding shows that it is inconsistent with the law of the conservation of energy. For, at the point where the nervous process is converted into mental activity, one sum of physical energy would disappear without being replaced by another sum of the same kind. As matter of fact no disappearance of energy takes place on account of the arising of a conscious state. The chain of psychical causation is not broken; its completeness no more suffers than if states of consciousness did not arise at all. Nor on the other hand does consciousness affect the sum of physical energy; it is hardly conceivable that it should even change the _direction_ of such energy (the sum supposably remaining constant, as is sometimes held), since to do this it must itself become a physical force. Moreover, if there is a relation of cause and effect between the brain and consciousness it would seem as if an interval of time must elapse between the process in the brain and the rising of the conscious state, a view to which the teachings of physiology lend no likelihood.