There can be no doubt that an important movement in psychology has arisen in recent times through the effort to approach the phenomena of mind from the experimental and physiological point of view. Different students of psychological science will estimate differently both the net result already reached by this effort and the promise of further additions to the sum of our knowledge from continued investigation of the same kind. Some writers have certainly indulged in extravagant claims as to the past triumphs of so called Physiological Psychology, and in equally extravagant expectations as to its future discoveries. On the other hand, a larger number, perhaps, have been inclined either to fear or to depreciate every attempt to mingle the methods, laws, and speculations of the physical sciences with the study of the human soul. These latter apparently anticipate that some discovery in the localization of cerebral function, or in psychometry, may jeopard the birthright of man as a spiritual and rational being. Or possibly they wish to regard the soul as separated, by nature and with respect to its modes of action, from the material body in such a way as to render it impossible to understand more of the one by learning more about the other. As a result of some years of study of the general subject, I express with considerable confidence the opinion that there is no ground for extravagant claims or expectations, and still less ground for any fear of consequences. In all cases of new and somewhat rankly growing scientific enterprises, it is much the better way to waive the discussion of actual or possible achievements, as well as of welcomed or dreaded revelations of new truth, and proceed at once to the business on hand. It is proposed in this book to follow this better way. It will be the task of the book itself to set forth the assured or alleged results of Physiological Psychology and this will be done at every step with such degree of assurance as belongs to the evidence hitherto attainable upon the particular subject discussed With declamation, either in attack or defence of the old psychology of the introspective method, etc., one may dispense without serious loss. The study of the phenomena of consciousness by tho method here proposed necessarily requires some acquaintance with a consider able circuit of sciences which are not usually all alike closely allied. The number of scholars who can form opinions with equal freedom and confidence in all of these sciences is very small. Moreover, since all psycho-physical laws are supposed as the very term indicates to govern the correlations of phenomena with phenomena of the nervous system, a peculiar mystery belong to much of the domain withiri which psycho-physical science i compelled to move. These facts may fitly, on tho one hand, excite caution in the writer and, on the other hand, exciiso him for many inevitable failures to set forth with perfect definition and confidence the conclusions he has to propose. Much will bo said thai must be accepted as provisional, as only probably true. Much room must also be made for conjecture and speculation. What IB most important, however, is that conjecture should not boptit forth as ascertained fact, or speculation as unquestioned law. It would have been a great assistance to mo if I had had more predecessors in the path which I am to take. But with tho exception of Wundts masterly work Grundzttye der phyxiohxjiwhtn Psychologies second edition in 1880, no ouo book han attempted to cover, even in a summary way, the entire ground...