A theory in natural science is a comprehensive formula or doctrine which describes and correlates in a unified abstract form of statement the general determining factors of some special group of natural facts. It is at once inclusive, realistic and understandable. If a theoretical statement holds good, the existence and characteristics of many individual events can be inferred deductively from it. It thus gives a logical basis for empirical fact. But it is based on experience of nature, and must conform to the facts of nature; these are primary, and a valid theory, no matter how broad it may be, is to be considered as itself a statement of fact, i.e., a true description of some condition which has a real and stable existence in the natural world. In its scientific usage, the term “fact” refers primarily to some datum of experience having the special character of permanence or regular recurrence; this is why scientific facts are verifiable; in this respect they differ from the passing individual events of history. Any scientific theory presupposes the stability and unequivocality of the natural facts. Hence if a theory is sound, it furnishes a reliable basis for prediction, discovery and invention. Scientific or technological creation then becomes possible; modern engineering and medicine could never have become what they are without a background of theoretical physics and biology. Obviously any natural science must always have at its disposal an ample body of observational and experimental data, but in most cases the work of collecting and verifying these is carried out under the guidance of theory.