Eliminating the future tense from the statement, however, I can agree with it. The instrument of translation from the mental to the physical, and back from the physical to the mental, is undoubtedly the brain, but as to how the translation is accomplished, I venture to say, we have not the inkling of an idea. Nevertheless, hints which may gradually lead towards a partial understanding of psycho-physical processes may be gained by study of exceptional cases: for such study is often more instructive than continued scrutiny of the merely normal. The fact of human consciousness, though it raises the problem to a high degree of conspicuousness, by no means exhausts the difficulty; for it is one which faces us in connexion with every form of life. The association of life with matter, and of mind with life, are problems of similar order, and a glimmering of understanding of the one may be expected to throw light upon the other. But until we know more of the method by which the simplest and most familiar psycho-physical interaction occurs--until we know enough to see how the gulf between two apparently different Modes of Being is bridged--it is safest to observe and accumulate facts, and to be very chary of making more than the most tentative and cautious of working hypotheses. For to frame even a tentative hypothesis, of any helpful kind, may require some clue which as yet we do not possess. I have been struck by the position taken by Dr. Chalmers Mitchell in his notable small book _Evolution and the War_, the early chapters of which, on Germany of the past and present, I would like unreservedly to commend to the reader. Indeed, commendation of a friendly and non-patronising kind may well extend to the whole book, although it must be admitted that here and there mere exposition of Darwinism is suspended, and difficult and debatable questions are touched upon. On these questions I would not like to be understood as expressing a hasty opinion, either against or for the views of the author. The points at issue between us are more or less fine-drawn, and cannot be dealt with parenthetically; nor do I ever propose to deal with them in a controversial manner. The author, as a biologist of fame, is more than entitled to such expression of his own views as he has cared to give. I quote with admiration, not necessarily with agreement, a few passages from the part dealing with the relation between mind and matter, and especially with the wide and revolutionary difference between man and animal caused by either the evolution or the incoming of free and conscious Choice.