Whoever has even an inkling of his character will entertain no doubts. At that time, three hundred years ago, the materials for a magnificent scene, "one world _versus_ the other," lay ready. Only one condition was wanting, the moral courage of the hero. The lack of this one factor spoilt the final act for the history of that time. The fine ethical feelings of later generations have had to be propitiated by improvising a legend iridescent with beautiful colours. CHAPTER VII PROBLEMS Questions of the Future.--Problem of Three Bodies.--Conception of Approximation.--Object of Mechanics.--Simplicity of Description.--Limits of Proof.--Reflections about the Circle.--From the History of Errors.--Causalities.--Relativity on a Physiological Basis.--Physicists as Philosophers. WE spoke of the objects and problems of science in general, and touched on certain recurrent questions with which reputed men of science are confronted from time to time, so that we may ascertain their opinions about immediate as well as more remote aims, and about worthy objects and those within reach. "Such stimuli," said Einstein, "may be quite interesting inasmuch as they sharpen the appetite of the public for the works of investigators, and give the latter the opportunity of making wider circles acquainted with their plans. Yet the value of their suggestions must not be overrated, when they are directed at giving trustworthy information about the future lines of development of science. Every scientist, in working out his own research, gravitates to particular points on the boundary which separates the known from the unknown, and becomes inclined to take his particular perspective from these points. It must not, however, be expected that these individual aspects will form a complete picture, and will indicate the only paths along which science can or will advance." "May I suggest, Professor," I answered, "that we select certain answers that have been given to these recurrent questions for discussion? I have brought along a whole series of them; it would be of value to know what attitude you take up towards some of the statements that have been made about future possibilities." Einstein acquiesced, and so I read out a number of expressions of opinion, given by eminent authorities, particularly in natural science and mathematics. They came under the heading, "The Future Revolution of Science." At the outset we encountered arguments by Bailhaud, the director of the Paris Observatory; he dealt with the so-called "Problem of Three Bodies," and with "The Finitude or Infinitude of the Universe."