cynical pessimism as to both present and future, there can be no doubt. But neither may it be doubted that the movement was a necessity to bring the Antæaus of humanity back to the mother soil of experience, whence it derives its strength and very life, and to prevent it from losing itself in a substanceless vapor where its ideals and purposes become as thin and watery as the clouds towards which it aspires.
Out of this movement and as one of its best aspects came that organized, systematic, tireless study into the secrets of nature, which, counting nothing common or unclean, thought no drudgery beneath it, or rather thought nothing drudgery,-- that movement which with its results had been the great revelation given to the nineteenth century to make. In this movement psychology took its place, and in the growth of physiology which accompanied it I find the first if not the greatest occasion of the development of the New Psychology.
It is a matter in every one's knowledge that, with the increase of knowledge regarding the structure and functions of the nervous system, there has arisen a department of science known as physiological psychology, which has already thrown great light upon psychical matters. But unless I entirely misapprehend the popular opinion regarding the matter, there is very great confusion and error in this opinion, regarding the relations of this science to psychology. This opinion, if I rightly gather it, is, that physiological psychology is a science which does, or at least claims to, explain all psychical life by reference to the nature of the nervous system. To illustrate: very many professed popularizers of the results of scientific inquiry, as well as laymen, seem to think that the entire psychology of vision is explained when we have a complete knowledge of the anatomy of the retina, of its nervous connection with the brain, and of the centre in the latter which serves for visual functions; or that we know all about memory if we can discover that certain brain cells store up nervous impressions, and certain fibres serve to connect these cells,-- the latter producing the association of ideas, while the former occasion their reproduction. In short, the commonest view of physiological psychology seems to be that it is a science which shows that some or all of the events of our mental life are physically conditioned upon certain nerve-structures, and thereby explains these events. Nothing could be further from the truth. So far as I know, all the leading investigators clearly realize that explanations of psychical events, in order to explain, must themselves be psychical and not physiological. However important such knowledge as that of which we have just been speaking may be for physiology, it has of itself no value for psychology. It tells simply what and how physiological elements serve as a basis for psychical acts; what the latter are, or how they are to be explained, it tells us not at all. Physiology can no more, of itself, give us the what, why, and how of psychical life, than the physical geography of a country can enable us to construct or explain the history of the nation that has dwelt within that country. However important, however indispensable the land with all its qualities is as a basis for that history, that history itself can be ascertained and explained only through historical records and historic conditions. And so psychical events can be observed only through psychical means, and interpreted and explained by psychical conditions and facts.
What can be meant, then, by saying that the rise of this physiological psychology has produced a revolution in psychology? This: that it has given a new instrument, introduced a new method,-- that of experiment, which has supplemented and corrected the old method of introspection. Psychical facts still remain psychical, and are to be explained through psychical conditions; but our means of ascertaining what these facts are and how they are conditioned have been indefinitely widened. Two of the chief elements of the method of experiment are variation of conditions at the will and under the control of the experimenter, and the use of quantitative measurement. Neither of these elements can be applied through any introspective process. Both may be through physiological psychology. This starts from the well-grounded facts that the psychical events known as sensations arise through bodily stimuli, and that the psychical events known as volitions result in bodily movements; and it finds in these facts the possibility of the application of the method of experimentation. The bodily stimuli and movements may be directly controlled and measured, and thereby, indirectly, the psychical states which they excite or express.
There is no need at this day to dwell upon the advantages derived in any science from the application of experiment. We know well that it aids observation by indefinitely increasing the power of analysis and by permitting exact measurement, and that it equally aids explanation by enabling us so to vary the constituent elements of the case investigated as to select the indispensable. Nor is there need to call attention to the especial importance of experiment in a science where introspection is the only direct means of observation. We are sufficiently aware of the defects of introspection. We know that it is limited, defective, and often illusory as a means of observation, and can in no way directly explain. To explain is to mediate; to connect the given fact with an unseen principle; to refer the phenomenon to an antecedent condition ,-- while introspection can deal only with the immediate present, with the given now. This is not the place to detail the specific results accomplished through this application of experiment to the psychological sphere; but two illustrations may perhaps be permitted: one from the realm of sensation, showing how it has enabled us to analyze states of consciousness which were otherwise indecomposable; and the other from that of perception, showing how it has revealed processes which could be reached through no introspective method.
It is now well known that no sensation as it exists in consciousness is simple or ultimate. Every color sensation, for example, is made up by at least three fundamental sensory quales, probably those of red, green, and violet; while there is every reason to suppose that each of these qualities, far from being simple, is compounded of an indefinite number of homogeneous units. Thus the simplest musical sensation has also been experimentally proved to be in reality not simple, but doubly compound. First, there is the number of qualitatively like units constituting it which occasion the pitch of the note, according to the relations of time in which they stand to each other; and second, there is the relation which one order of these units bears to other secondary orders, which gives rise to the peculiar timbre or tone-color of the sound; while in a succession of notes these relations are still further complicated by those which produce melody and harmony. And all this complexity occurs, be it remembered, in a state of consciousness which, to introspection, is homogeneous and ultimate. In these respects physiology has been to psychology what the microscope is to biology, or analysis to chemistry. But the experimental method has done more than reveal hidden parts, or analyze into simpler elements. It has aided explanation, as well as observation, by showing the processes which condition a psychical event. This is nowhere better illustrated than in visual perception. It is already almost a commonplace of knowledge that, for example, the most complex landscape which we can have before our eyes, is, psychologically speaking, not a simple ultimate fact, nor an impression stamped upon us from without, but is built up from color and muscular sensations, with, perhaps, unlocalized feelings of extension, by means of the psychical laws of interest, attention, and interpretation. It is, in short, a complex judgment involving within itself emotional, volitional, and intellectual elements. The knowledge of the nature of these elements, and of the laws which govern their combination into the complex visual scene, we owe to physiological psychology, through the new means of research with which it has endowed us. The importance of such a discovery can hardly be overestimated. In fact, this doctrine that our perceptions are not immediate facts, but are mediated psychical processes, has been called by Helmholtz the most important psychological result yet reached.
But besides the debt we owe Physiology for the method of experiment, is that which is due her for an indirect means of investigation which she has put within our hands; and it is this aspect of the case which has led, probably, to such misconceptions of the relations of the two sciences as exist. For while no direct conclusions regarding the nature of mental activities or their causes can be drawn from the character of nervous structure or function, it is possible to reason indirectly from one to the other, to draw analogies and seek confirmation. That is to say, if a certain nervous arrangement can be made out to exist, there is always a strong presumption that there is a psychical process corresponding to it; or if the connection between two physiological nerve processes can be shown to be of a certain nature, one may surmise that the relation between corresponding psychical activities is somewhat analogous. In this way, by purely physiological discoveries, the mind may be led to suspect the existence of some