After this first point has been established our psychiatric interest will become even livelier. If a delusion is not to be got rid of by a reference to reality, no doubt it did not originate from reality either. Where else did it originate? There are delusions of the most varied content: why in our case is the content of the delusion jealousy in particular? In what kind of people do delusions, and especially delusions of jealousy, come about? We should like to hear what the psychiatrist has to say about this; but at this point he leaves us in the lurch. He enters into only a single one of our enquiries. He will investigate the woman’s family history and will perhaps give us this reply: ‘Delusions come about in people in whose families similar and other psychical disorders have repeatedly occurred.” In other words, if this woman developed a delusion she was predisposed to it by hereditary transmission. No doubt this is something; but is it all we want to know? Was this the only thing that contributed to the causation of the illness? Must we be content to suppose that it is a matter of indifference or caprice or is inexplicable whether a delusion of jealousy arises rather than any other sort? And ought we to understand the assertions of the predominance of the hereditary influence in a negative sense as well – that no matter what experiences this woman’s mind encountered she was destined some time or other to produce a delusion? You will want to know why it is that scientific psychiatry will give us no further information. But my reply to you is ‘he is a rogue who gives more than he has.’ The psychiatrist knows no way of throwing more light on a case like this one. He must content himself with a diagnosis and a prognosis – uncertain in spite of a wealth of experience – of its future course.
Sigmund Freud – Introductory Lectures on Psychoanalysis p.310