I N T R O D U C T I O N T O V O L U M E 1 6 l x x x i i i was regarded by an American newspaper as an introduction to topics in theoretical physics suitable for its readers. The same need to find language in which to express his ideas is apparent also in Einstein’s correspondence, in which, as in other vol- umes, we find him needing to discuss physics in relatively down-to-earth terms. One might expect Einstein, faced with so many demands on his time, to merely repeat the same pithy summaries of his theories over and over again. But the diffi- culty was that he had quite varied audiences. Philosophers sought clarification of his ideas in terms useful to their systems. Amateur scientists looked for confirma- tion of the worth of their ideas, and these were often at a high level, as in the letters in this volume from eccentric Irish inventor Edward Hutchinson Synge, brother of John Lighton Synge, later to be the leading exponent of general relativity during its low watermark period. E. H. Synge’s ideas were ignored during his lifetime, but are now recognized as being a prescient and remarkably detailed prefiguring of near-field scanning microscopy (McMullan 1990 and Novotny 2007). Einstein saw that his ideas were not to be dismissed lightly and devoted considerable time to his replies to Synge. Another, very typical example in this volume is Doc. 305, in which Einstein re- sponds to an amateur interested both in the increasing evidence for the redshifting of light from the galaxies and in experimental confirmation of Mach’s principle. Although letters such as these were addressed to people with a reasonable concep- tual understanding of topics in physics, Einstein felt obliged to formulate his re- plies in a non-mathematical, and generally even non-numerical, way. Even though expounding on these issues required calculation on his part, such as whether frame dragging is detectable by terrestrial experiments, his correspondents benefited from a relatively accessible presentation in his replies. This generous impulse of Einstein, to give unknown correspondents a serious reading, had paid off for him in the case of Satyendra N. Bose’s approach to quantum statistics (see, e.g., Vol. 14, Doc. 278 for Einstein’s translation of Bose’s paper) and would continue to mark his attitude in later years as, for instance, when prompted a few years later by an ama- teur to publish his work on gravitational lensing (see Renn and Sauer 1997). Einstein was perfectly prepared to carry on his scientific outreach one solitary thinker at a time. Old friends, casual acquaintances and complete strangers alike might propound their sometimes cranky ideas. Remarkably, Einstein found something original to say to many of them, even if it was just in the correction of some flaw, serious or minor, in their conceptions.