I am now going to ask you to consider these phenomena. But you will object, with annoyance: “There are so many tremendous puzzles both in the wide world and in the narrower life of the soul, so many mysteries in the field of mental disorder which demand and deserve explanation, that it really seems frivolous to waste labour and interest on these trifles. If you could explain to us how it is possible for anyone with sound sight and hearing, in broad daylight, to see and hear things which do not exist, or how anyone can suddenly believe that his nearest and dearest are persecuting him, or can justify with the most ingenious arguments a delusion which would seem nonsensical to any child, then we might be willing to take psycho-analysis seriously. But if psycho-analysis cannot occupy us with anything more interesting than the question why a speaker uses a wrong word or why a _Hausfrau_ mislays her keys and similar trivialities, then we shall find something better to do with our time and our interest.” My reply is: Patience! Your criticism is not on the right track. It is true that psycho-analysis cannot boast that it has never occupied itself with trifles. On the contrary, the material of its observations is usually those commonplace occurrences which have been cast aside as all too insignificant by other sciences, the refuse, so to speak, of the phenomenal world. But in your criticism are you not confounding the magnitude of a problem with the conspicuous nature of its manifestations? Is it not possible, under certain conditions and at certain times, for very important things to betray themselves in very slight indications? I could easily cite many instances of this. What slight signs, for instance, convey to the young men in my audience that they have gained a lady’s favour? Do they expect an explicit declaration, a passionate embrace, or are they not content with a glance which is almost imperceptible to others, a fleeting gesture, a handshake prolonged by a second? Or suppose you are a detective engaged in the investigation of a murder, do you actually expect to find that the murderer will leave his photograph with name and address on the scene of the crime? Are you not perforce content with slighter and less certain traces of the person you seek? So let us not undervalue small signs: perhaps from them it may be possible to come upon the tracks of greater things. Besides, I think as you do that the larger problems of the world and of science have the first claim on our interest. But on the whole it avails little to form a definite resolution to devote oneself to the investigation of this or that great problem. One is then often at a loss how to set about the next step. In scientific work it is more profitable to take up whatever lies before one whenever a path towards its exploration presents itself. And then, if one carries it through thoroughly, without prejudice or pre-conceptions, one may, with good