I quite acknowledge that there is an excuse for this defect in your previous training. There is no auxiliary philosophical science that might be of service to you in your profession. Neither speculative philosophy nor descriptive psychology, nor even the so-called experimental psychology which is studied in connection with the physiology of the sense-organs, as they are taught in the schools, can tell you anything useful of the relations existing between mind and body, or can give you a key to comprehension of a possible disorder of the mental functions. It is true that the psychiatric branch of medicine occupies itself with describing the different forms of recognizable mental disturbances and grouping them in clinical pictures, but in their best moments psychiatrists themselves are doubtful whether their purely descriptive formulations deserve to be called science. The origin, mechanism, and interrelation of the symptoms which make up these clinical pictures are undiscovered: either they cannot be correlated with any demonstrable changes in the brain, or only with such changes as in no way explain them. These mental disturbances are open to therapeutic influence only when they can be identified as secondary effects of some organic disease. This is the lacuna which psycho-analysis is striving to fill. It hopes to provide psychiatry with the missing psychological foundation, to discover the common ground on which a correlation of bodily and mental disorder becomes comprehensible. To this end it must dissociate itself from every foreign preconception, whether anatomical, chemical, or physiological, and must work throughout with conceptions of a purely psychological order, and for this very reason I fear that it will appear strange to you at first. For the next difficulty I shall not hold you, your training or your mental attitude, responsible. There are two tenets of psycho-analysis which offend the whole world and excite its resentment; the one conflicts with intellectual, the other with moral and æsthetic, prejudices. Let us not underestimate these prejudices; they are powerful things, residues of valuable, even necessary, stages in human evolution. They are maintained by emotional forces, and the fight against them is a hard one.