admitted sort of object whose habits of coexistence and succession and relations with organic conditions form an entirely definite subject of research. Cannot philosophers and biologists both become 'psychologists' on this common basis? Cannot both forego ulterior inquiries, and agree that, provisionally at least, the mental state shall be the ultimate datum so far as 'psychology' cares to go? If the 'scientific monists' would only agree to say nothing of the states being produced by the integration and differentiation of 'psychic units' and the 'transcendental metaphysicians' agree to say nothing of their being acts of spiritual entities developing according to laws of their own, peace might long reign, and an enormous booty of natural laws be harvested in with comparatively no time or energy lost in recrimination and dispute about first principles. My own volumes are indeed full of such recrimination and dispute, but these unfortunate episodes are for the most part incidental to the attempt to get the undivided 'mental state' once for all accepted by my colleagues as the fundamental datum for their science. To have proposed such a useful basis for united action in psychology is in my own eyes the chief originality and service of the book; and I cannot help hoping that Professor Ladd may himself yet feel the force of the considerations now urged. Not that to-day we have a 'science' of the correlation of mental states with brain-states; but that the ascertainment of the laws of such correlation forms the program of a science well limited and defined. Of course, when such a science is formed, the whole body of its conclusions will fall a prey to philosophical reflection, and then Professor Ladd's 'real being' will inevitably have the best possible chance to come to its rights.
One great reason why Professor Ladd cares so little about setting up psychology as a natural science of the correlations of mental with cerebral events, is that brain-states are such desperately inaccessible things. I fully admit that any exact account of brain states is at present far beyond our reach; and I am surprised that Professor Ladd should have read into my pages the opinion that psychology as a natural science must aim at an account of brain states exclusively, as the correlates of states of
