attempt has not been successful; it is not possible to transfer the concepts of that most developed portion of physics to the other domains, each of which requires the formation of its own special concepts.[1] The three units, time, space and mass, are not sufficient for the definition of physical quantities except in mechanics; to these a special unit must be added for each domain of phenomena. We have been so long accustomed to attempt to translate all objective phenomena into terms of touch and the muscle sense, and have gotten into such hopeless difficulty that with such a point of view it is quite intelligible that so many should absolutely deny the possibility of psychological measurements. As soon as with Ostwald we look upon the relations of the various classes of objective phenomena as those of equivalence and not of identity, the close interrelation between psychology and physics becomes comprehensible. The two sciences divide the field of immediately experienced facts. Each phenomenon has an objective or physical side and a subjective or mental side, the two being intimately related and sometimes indistinguishable. We can compare physical, or objective, phenomena directly with mental, or subjective, phenomena. Experimental psychology has in great part to do with such comparisons; in a large part of the work the experiments are psychophysical.
Rejection of Metaphysics as a Basis.–We have become so accustomed to certain hypotheses that it is difficult for us to look at matters as they are actually given us. From the study of the objective phenomena we have constructed our physical world, in which we find other beings to whom we are inclined to attribute conscious phenomena like our own. By a series of conclusions we suppose that their nervous systems are most closely connected with such phenomena, and it then becomes an object to determine what conscious phenomena are connected with the activities of the nervous system. Thereupon we turn the matter around again and try to look at our own facts of immediate experience as if they were some one else’s. When we make a psychological experiment, instead of comparing the
