Some of the simple experiments of physiological psychology illustrate the point to which I have been referring. If one stays in a dark room until the eye has adapted itself to the absence of light, and then for a second subjects oneself to a ray of coloured light, a sensation of illumination will be received, although it is impossible to recognise the quality of the illumination; something has been perceived, but what the something is cannot be apprehended unless the stimulation lasts a definite time. In the same way every scientific discovery, every technical invention, every artistic creation passes through a preliminary phase of indistinctness. The process is similar to the series of impressions that would be got as a statue was gradually unwrapped from a series of swathings. The same kind of sequence occurs, although, perhaps, in a very brief space of time, when one is trying to recall a piece of music. Every thought is preceded by a kind of half-thought, a condition in which vague geometrical figures, shifting masks, a swaying and indistinct background hover in the mind. The beginning and the end of the whole process, which I may term “clarification,” are what take place when a short-sighted person proceeds to look through properly adapted lenses. Just as this process occurs in the life of the individual (and he, indeed, may die long before it is complete), so it occurs in history. Definite scientific conceptions are preceded by anticipations. The process of clarification is spread over many generations. There were ancient and modern vague anticipations of the theory of Darwin and Lamarck, anticipations which we are now apt to overvalue. Mayer and Helmholtz had their predecessors, and Goethe and Leonardo da Vinci, perhaps two of the most many-sided intellects known to us, anticipated in a vague way many of the conclusions of modern science. The whole history of thought is a continuous “clarification,” a more and more accurate description or realisation of details. The enormous number of stages between light and darkness, the minute gradations of detail that follow each other in the development of thought can be realised best if one follows historically some complicated modern piece of knowledge, such as, for instance, the theory of elliptical functions.