Now imagine for a moment that you were present at a lecture in history instead of in psychiatry, and that the lecturer was dealing with the life and conquests of Alexander the Great. What reason would you have to believe what he told you? The situation would appear at first sight even more unsatisfactory than in the case of psycho-analysis, for the professor of history had no more part in Alexander’s campaigns than you yourselves; the psycho-analyst at least informs you of matters in which he himself has played a part. But then we come to the question of what evidence there is to support the historian. He can refer you to the accounts of early writers who were either contemporaries or who lived not long after the events in question, such as Diodorus, Plutarch, Arrian, and others; he can lay before you reproductions of the preserved coins and statues of the king, and pass round a photograph of the mosaic at Pompeii representing the battle at Issus. Yet, strictly speaking, all these documents only prove that the existence of Alexander and the reality of his deeds were already believed in by former generations of men, and your criticism might begin anew at this point. And then you would find that not everything reported of Alexander is worthy of belief or sufficiently authenticated in detail, but I can hardly suppose that you would leave the lecture-room in doubt altogether as to the reality of Alexander the Great. Your conclusions would be principally determined by two considerations: first, that the lecturer could have no conceivable motive for attempting to persuade you of something which he did not himself believe to be true, and secondly, that all the available authorities agree more or less in their accounts of the facts. In questioning the accuracy of the early writers you would apply these tests again, the possible motives of the authors and the agreement to be found between them. The result of such tests would certainly be convincing in the case of Alexander, probably less so in regard to figures like Moses and Nimrod. Later on you will perceive clearly enough what doubts can be raised against the credibility of an exponent of psycho-analysis.