In science, whether pure or applied, a theory is both the culmination of a research cycle and a guide to further research. In applied science theories are, in addition, the basis of systems of rules prescribing the course of optimal practical action. On the other hand in the arts and crafts theories are either absent or instruments of action alone. Not whole theories, though, but just their peripheral part: since the low level consequences of theories can alone come to grips with action, such end results of theories will concentrate the attention of the practical man. In past epochs a man was regarded as practical if, in acting, he paid little or no attention to theory or if he relied on worn-out theories and common knowledge. Nowadays a practical man is one who acts in obeyance to decisions taken in the light of the best technological knowledge — not scientific knowledge, because most of this is too remote from or even irrelevant to practice. And such a technological knowledge, made up of theories, grounded rules, and data, is in turn an outcome of the application of the method of science to practical problems (see Sec. 1.5).