There can be no doubt but that Einstein has a claim to be heard as an authority on these questions. There are few in the realm of the learned in whose faces it is so clearly manifest that they are called to excite a desire for knowledge by means of the living word, and to satisfy this desire. If great audiences assemble around him, if so many foreign academies open their arms to him to make him their own, these are not only signs of a magnetic influence that emanates from the famous discoverer, but they are indications that he is far famed as a teacher with a captivating personality. Let us consider what this signifies in his profession. Philosophers, historians, lawyers, doctors, and theologians have at their disposal innumerable words which they merely need to pronounce to get into immediate contact with their audiences. In Einstein's profession, theoretical physics, man disappears; it leaves no scope for the play of emotion; its implement mathematics--and what an instrument it is!--bristles with formal difficulties, which can be overcome only by means of symbols and by using a language which has no means of displaying eloquence, being devoid of expression, emotion, and regular periods. Yet here we have a physicist, a mathematician, whose first word throws a charm over a great crowd of people, and who extracts from their minds, so to speak, what, in reality, he alone works out before them. He does not adhere closely to written pages, nor to a scheme which has been prepared beforehand in all its details; he develops his subject freely, without the slightest attempt at rhetoric, but with an effect which comes of itself when the audience feels itself swept along by the current. He does not need to deliver his words passionately, as his passion for teaching is so manifest. Even in regions of thought in which usually only formulæ, like glaciers, give an indication of the height, he discovers similes and illustrations with a human appeal, by the aid of which he helps many a one to conquer the mountain sickness of mathematics. His lectures betray two factors that are rarely found present in investigators of abstract subjects; they are temperament and geniality. He never talks as if in a monologue or as if addressing empty space. He always speaks like one who is weaving threads of some idea, and these become spun out in a fascinating way that robs the audience of the sense of time. We all know that no iron curtain marks the close of Einstein's lecture; anyone who is tormented by some difficulty or doubt, or who desires illumination on some point, or has missed some part of the argument, is at liberty to question him. Moreover, Einstein stands firm through the storm of all questions. On the very day on which the above conversation took place he had come straight from a lecture on four-dimensional space, at the conclusion of which a tempest of questions had raged about him. He spoke of it not as