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 PROF. MUNSTEEBEBG AS CRITIC OF CATEGORIES. 211 a most valuable method, but its value is limited to the parti- cular end which it has in view, and to which its categories confine it. That end is to grasp the relationships of the complex of ideas of perception which make up the object world of inner life. Just as the physicist proceeds by con- ceiving his object world as resoluble into atoms, so the psychologist treats his as resoluble into sensations. Both points of view are abstract, and shut out other aspects of reality. But each is necessary as the instrument by which alone clear knowledge is to be attained. So much for the relation of psychology to the facts of mental life. In the next chapter Prof. Miinsterberg examines the relationship of psychology to physiology. He begins by stating that while external phenomena can be directly apprehended by a plurality of persons, the pheno- mena of internal life are confined to the consciousness of the person to whom they belong, and can come into no other consciousness. All description for the purpose of communication is therefore in terms of some external phenomenon which is connected in our own experience with the internal one which it is sought to describe, and may suggest a similar connexion in the experience of another person. In fact, that there is an external pheno- menon corresponding to every internal experience, must be the postulate of every attempt at a science of psychology. But this is not all. Causal connexion properly so-called is an affair of the external world, where every change not only has its cause but is quantitatively equivalent to that cause. From the nature and quantity of the effect we can determine, therefore, not only the nature but the quantity of the cause. Now, in the world of inner sense, this is not so. The sensation which succeeds is not the sensation which has gone before in another form. The latter had perished wholly before the former came into existence. Thus we are shut out, not only from a causal view of sensa- tions, but from a quantitative view. We can supply the double deficiency only by turning to the physical processes which accompany sensation, and this is why, from the point of view of science, they are of such immense import- ance. They represent, it is true, only another aspect of the whole, but the new aspect is one in which we reach quanti- ties and causes, the two conceptions most fruitful for the purposes of scientific knowledge. The demand made on psychology is " to give causal explanations ; it can do so only if it replaces the psychical objects by constructions which are themselves conceived in analogy with physical