It is necessary that we should clearly understand what is meant by the psychological conception of mental disorder, the relation it bears to the physiological conception, and the nature of its aims and methods. The consideration of these problems involves some preliminary acquaintance with the fundamental propositions upon which modern science is based, and inevitably leads to questions which are, strictly speaking, beyond the limits of our subject. But as no clear thinking in psychology is possible unless precise notions about these elementary matters have been previously acquired, we may perhaps be pardoned for temporarily trespassing upon the domain of philosophy.

It has been pointed out that modern science is attacking the problem of mental disorder along two different routes. The psychological conception treats the phenomena as states of mind, whereas the physiological conception treats them merely as manifestations of changes occurring in the brain. Now in this statement we are confronted with certain terms which, although loosely and erroneously used in ordinary language, have nevertheless a precise and definite signification. These terms are ‘science’, ‘mind’, ‘brain’, ‘physiology’, and ‘psychology’.

Everybody is aware that science is a method of acquiring knowledge, but everybody is not aware that this method is always one and the same, whatever the subject may be to which it is being applied. The scientist who devotes himself to psychology proceeds in exactly the same manner as the scientist who devotes himself to chemistry.