  Weyl analyzed the foundations of both the general theory of  relativity and the theory of quantum mechanics. For both theories, he provided a coherent exposition of the mathematical structure of the  theory, elegant characterizations of the entities and laws postulated by the theory and a lucid account of how these postulates explain the most significant, more directly observable, lower-level phenomena. In both cases, he was also concerned with the constructive aspects of  the theory, that is, with the extent to which the higher-level  postulates of the theory are necessary. 