However, there are always some people to whom the possibility of a new addition to knowledge will prove an attraction strong enough to survive all such inconveniences. If there are any such among you who will appear at my second lecture in spite of my words of warning, they will be welcome. But all of you have a right to know what these inherent difficulties of psycho-analysis are to which I have alluded. First of all, there is the problem of the teaching and exposition of the subject. In your medical studies you have been accustomed to use your eyes. You see the anatomical specimen, the precipitate of the chemical reaction, the contraction of the muscle as the result of the stimulation of its nerves. Later you come into contact with the patients; you learn the symptoms of disease by the evidence of your senses; the results of pathological processes can be demonstrated to you, and in many cases even the exciting cause of them in an isolated form. On the surgical side you are witnesses of the measures by which the patient is helped, and are permitted to attempt them yourselves. Even in psychiatry, demonstration of patients, of their altered expression, speech and behaviour, yields a series of observations which leave a deep impression on your minds. Thus a teacher of medicine acts for the most part as an exponent and guide, leading you as it were through a museum, while you gain in this way a direct relationship to what is displayed to you and believe yourselves to have been convinced by your own experience of the existence of the new facts.