Since the beginning of the twentieth century, science has gained notably in expertness, and lost notably in authority. We are bombarded with inventions; but if we ask the inventors what they have learned of the depths of nature, which somehow they have probed with such astonishing success, their faces remain blank. They may be chewing gum; or they may tell us that if an aeroplane could only fly fast enough, it would get home before it starts; or they may urge us to come with them into a dark room, to hold hands, and to commune with the dear departed. Practically there may be no harm in such a division of labour, the inventors doing the work and the professors the talking. The experts may themselves be inexpert in verbal expression, or content with stock phrases, or profoundly sceptical, or too busy to think. Nevertheless, skill and understanding are at their best when they go together and adorn the same mind. Modern science until lately had realised this ideal: it was an extension of common perception and common sense. We could trust it implicitly, as we do a map or a calendar; it was not true for us merely in an argumentative or visionary sense, as are religion and philosophy. Geography went hand in hand with travel, Copernican astronomy with circumnavigation of the globe: and even the theory of evolution and the historical sciences in the nineteenth century were continuous with liberal reform: people saw in the past, as they then learned to conceive it, simply an extension of those transformations which they were witnessing in the present. They could think they knew the world as a man knows his native town, or the contents of his chest of drawers: nature was our home, and science was our home knowledge. For it is not intrinsic clearness or coherence that make ideas persuasive, but connection with action, or with some voluminous inner response, which is readiness to act. It is a sense of on-coming fate, a compulsion to do or to suffer, that produces the illusion of perfect knowledge.