PSYCHOLOGY has been defined, from time immemorial, as the science of mind. This definition, which agrees well with the etymolog ical meaning of the word gpsychology,° may be accepted by the modern psychologist, provided that its two terms (gscience° and °mind))) are themselves correctly defined. This is an import ant point: for it is dear that the terms of a definition must be mutually compatible. If we speak of a °science of we must define the word °science° in such a way as to make it cover the field of mental phenomena, and we must define the word °mind° in such a way as to make it amenable to scientific treatment. To speak of a °science of mind° when one meant by °mind° something outside and beyond the reach of scientific method would be absurd.
The general characteristic of science is just its method. All science proceeds by observa tion; but the term science is not usually applied to any body of knowledge until it has reached the stage of experiment,— the stage, that is, at which its observations can be taken under stand conditions, can be repeated, isolated and varied at will. Further, the term science is usu ally reserved for classified and coherent knowl edge; for experimental observations that can be grouped together, as illustrative of some natural law or uniformity. One of the tests of a science, from this point of view, is its capacity to predict, on the basis of natural law, what will happen, under given conditions, in the individual case. Yet again, the term science, in its strictest sense, is reserved for explanatory knowledge: the observations must not only be accurate, must not only fall into groups, under general laws, and thus render it possible to predict the outcome of new observations, but must also be linked with and referred to their own causes or conditions in the world at large. Experiment, uniformity and explanation,—these are the three tests which psychology must satisfy, if it is to rank as a science among the sciences.
Within the scope of these general require ments, the sciences are differentiated by differ ences of their subject matter. Now the subject matter of psychology is mind: not mind as popularly understood, perhaps, but mind which is accessible to experiment, mind whose uni formities we can discover, mind whose proc esses we can explain. Descartes, when he set to work to reconstitute philosophy, took his stand upon the proposition "I think" It is a long 250 years since Descartes gave his 'Dis course) to the world; and what, to him, was metaphysical speculation has now become an unquestioned belief of common sense. We
all tend to look at mind as Descartes looked at it; we all tend to say eI think," and so to separate the "I," as active thinker, from the "thought" which is the result of the I's activity. Psychology cannot recognize this distinction. The datum for a psychology, if it is to be a scientific psychology, is rather "Thought goes on:" "there are processes of thought in the world." And so the subject matter of psychol ogy is thought just as thought exists, thought looked at for its own sake and interest. Or rather, since mind is a wider term than thought, and covers the facts of feeling and of will as well as the facts of intellect, the subject matter of psychology is mental process: the whole tangle of thoughts, memories, fancies, feelings, emotions, resolves, desires, aspirations, that make up our mind as we look at it from mo ment to moment. The problem of psychology is to bring order into this chaos, to reduce the infinite variety of our immediate experience to a coherent and manageable system.
If it be asked how precisely this material differs from the material upon which other sciences are working, what is the criterion that differentiates the subject matter of psychology from the subject matter of physics, the reply is simple. Mental process, the material of psychol ogy, is confined to the exepricnce of a single individual ; psychological experience is unshared experience. The universe of nature is common to all of us; the universe of mind is open but to one,— to each man his own. The laws of physics and of physiology take no account of the individual: sound is a certain motion of the air waves, digestion is a certain sequence of chemical changes; and the phenomena of sound and of digestion are treated abstractly, so that we can conceive of a deaf man as understand ing acoustics, or of an anesthetic patient as familiar with physiological chemistry. Psy chology, on the other hand, deals with the sound as we hear it, with the hunger and thirst that we feel; and these experiences, however ac curately we may express them in words, are, as experiences, incommunicable.