Realism and instrumentalism provide two alternative interpretations of science and of factual knowledge in general. According to realism such knowledge is descriptive of features of the universe. According to the Aristotelian philosophy, which was the accepted basis for physical reasoning throughout the later Middle Ages, motion was to be understood as the actualization of a potentiality inherent in an object. It is very important to realize this complex character of the situation, for otherwise one will be satisfied too early and too easily. The emphasis upon the absence of predictability is not satisfactory either. For way of speaking would again suggest that we could perhaps predict better if we only knew more about things that exist in the universe, whereas Bohr's suggestion denies that there are things whose detection would make our knowledge more definite. The empirical adequacy of the proposed solution is shown by such phenomena as the natural line breadth, which in some cases may be quite considerable.