Professor Ladd's opinion that Professor James's conception of psychology excludes the entire domain of physiological psychology, is due to his failing to note the force of "last" in the passage he quotes. Professor James says the last word of a psychology that seeks to be clear using "last" in the sense of "ultimate" — will consist in "a blank unmediated correspondence, term for term, of the succession of states of consciousness with the succession of total brain-processes." In other words, if the problem of psychology as natural science is ever completely solved, its solution will consist in the discovery of this blank unmediated correspondence. In the preface, he makes it unnecessary for us to infer his meaning. He says in so many words that he has regarded the mere laws of coexistence of our passing thoughts as integers with brain-states, as "the ultimate laws" of psychology. He has certainly nowhere hinted that any derivative laws which have already been, or may hereafter be established have no place in the science. Professor Ladd is right when he says that Professor James's conception excludes introspective psychology as explanatory science. But is it explanatory science from Professor Ladd's own point of view? Not unless he believes that ideas or states of consciousness continue to exist after we cease to be conscious of them. An idea that does not exist cannot explain anything. Those who believe that they only exist when we are conscious of them must limit introspective psychology to description and classification, no matter how they conceive the science.
But he contends that Professor James does not adhere to his conception of psychology, that he " postulates some of those deeper lying entities and then puts them through a course of conjectural processes in order to explain other [conscious] processes which are not conjectural, but are indubitably known to exist." Professor James does assume the brain in the same practical common-sense way in which physiology assumes it, in the same way in which astronomy assumes the existence of the heavenly bodies and an objective space in which they move; do these sciences forsake the point of view of natural science in making these assumptions? But if the physical natural sciences can assume an external, material world, why may not psychology assume an internal, immaterial world? Because the physical sciences make no use whatever of their metaphysical assumptions. Whatever the metaphysical creed of the student of physical science, whether he be Berkleyan, Lotzian, Spencerian, Positivist, or Natural Realist, is a matter of absolute indifference to physical science. Physical science deals only with phenomena — makes no use of their metaphysical ground or condition. But that is precisely what Professor Ladd wants psychology to do. He insists that we have as good a right to say that the mind thinks as that the brain acts. So we have; and those who agree with Professor James will not
