On the hypothesis of scientific monism it is believed that they are different aspects of the same phenomena: that what objectively to the physical investigator are transformations of energy in the brain, are subjectively to the dog states of consciousness? Let us look into this hypothesis. Let us see whither it will lead us; and if it will in any way help us over some of our difficulties. But first let us pay a moment’s attention to the impatient exclamation which some may feel inclined to interpose, that this assumption of the ultimate identity of brain-energy and consciousness, the two being respectively the objective and subjective aspects of the same occurrences, does not in the least do away with the mystery of the matter. That the same two occurrences should have different aspects, objective and subjective, is, it will be said, just as mysterious as that two separate existences energy and consciousness should be associated together. Of course it is. I should be shallow and pretentious indeed if my object were by any _hocus pocus_ to attempt to hide the so-called mystery. _All_ ultimate facts are mysterious. The fall of a stone to the ground is to-day as mysterious as it was in the days before Newton; the phenomena of life, as mysterious as in the days before Darwin. Our advances in science and in thought may do away with minor mysteries, but they leave the great ultimate facts of nature as mysterious as before. The end of our explanations is always to bring us face to face with the inexplicable. Not, therefore, in any hope of doing away with an ultimate mystery do I suggest that we look into and follow out some implications of this so called identity hypothesis.