The title agrees with the circumstance to which this book owes its birth. And in undertaking to address itself to the circle of readers as to an audience, it promises much eloquence that came from Einstein's own lips, during hours of social intercourse, far removed from academic purposes and not based on any definite scheme intended for instruction. It will, therefore, be neither a course of lectures nor anything similar aiming at a systematic order and development. Nor is it a mere phonographic record, for this is made impossible if for no other reason than that whoever has the good fortune to converse with this man, finds every minute far too precious to waste it in snatching moments to take shorthand notes. What he has heard and discussed crystallizes itself in subsequent notes, and to some extent he relies on his memory, which would have to be extraordinarily lax if it managed to forget the essentials of such conversations. But these essentials could not be attained by clinging closely to the exact terms of utterance. This would be a gain neither for the scheme of the book nor for the reader who wishes to follow a great thinker in all the ramifications of his ideas. It must be reiterated that this book is intended neither as a textbook nor as a guide leading to a complete system of thought; nor, above all, is it in any way due to Einstein, nor desired by him. Any value and attraction of the book is rather to be sought in its kaleidoscopic nature, its loose connexion, which expresses a general meaning without being narrowed to pedantic limits by a restriction to literal repetition. It is just this absence of the method that is rightly demanded of a textbook, which may enable these conversations to pass on to the world a little of the pleasure which they originally gave me. Perhaps they will even be sufficient to furnish the reader with a picture of the eminent scientist, sufficient to give him a glimpse of his personality, without demanding a detailed study to secure this end. Even here I should like to state that the range of Einstein's genius extends much further than is generally surmised by those who have busied themselves only with the actual physical theory. It sends out rays in all directions, and brings into view wonderful cosmic features under his stimulus--features which are, of course, embedded in the very refractory mathematical shell of his physics which embraces the whole world. But only minds of the distant future, perhaps, will be in a position to realize that _all_ our mental knowledge is illuminated by the light of his doctrine.