cording to the definitions with which our psychologies start, it is indeed true that mind and matter must either interact or in some way correspond without interaction. If the psychologist asserts that each brain is a center for the creation of new energy or for interference with the configuration of a material system, he obviously subverts the principal generalizations of physical science. He doubtless has a right to do so, but in the same sense as the cow has a right to stop the locomotive engine. If, on the other hand, the psychologist modestly admits that mind does not affect the physical order, he runs counter to the principal generalization of biological science. If pleasure and pain, memory and forethought, are of no use in the struggle for organic survival, why should they ever have evolved?
It requires less temerity to question the theories of biology than to deny the laws of physics. The survival of the fit may be regarded as a truism rather than as a discovery, if we call that fit which does survive. But fitness of this kind is so protean in its manifestations in organic nature that the formula becomes somewhat vague. If an animal is inconspicuously colored, it is protective coloration and so useful; if conspicuously colored, it is directive coloration and so useful. It is somewhat difficult to guess the utility of the fantastic shape and color of each deep-sea fish that lives in perpetual darkness. Then there are admittedly correlated variations, by-products of evolution, diseases and the like; it may be that consciousness is that sort of thing. If some kinds of consciousness, as the sense of beauty, are of no use in the struggle for existence, all the rest may be equally useless—an efflorescence exhibited when there is friction due to lack of adjustment between the organism and its environment. Finally, and most plausibly, it may be argued that minds have evolved in answer to final causes, and that organic evolution must adopt the principles of psychology rather than prescribe to it.
The interactionist seems to be in a worse plight than the parallelist in the conflicts with our sister sciences, but the case is different before the court of common sense. The present writer can not conceive how the parallelist gets outside the limits of consciousness. Why does he want any thing to run parallel with the only thing he knows? He becomes at once a subjective idealist, and there may be no harm in that. But when the subjective idealist wants to live in a world with other men, he reinvents the distinctions that he had verbally obliterated. What he knows about the physical world is what his senses and the physicists tell him; if he likes to call it all consciousness or the unconscious, mind-stuff, will or God's thought, this may be emotionally stimulating, but no fact or law is thereby altered. The world may be God's thought, without in the least preventing the parallelist from thinking illogically.
