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.

If I miss not the tenor of points and counterpoints, a recent discussion in this journal has been a novelly natural transaction in behalf of a great question at which many philosophers have labored—What is the place of Mind? R. S. Lillie, an eminent physiologist has been working toward a philosophical justification of certain biological key-facts, and H. Heath Bawden, a pioneer naturalist in philosophy and psychology, has been urging a physiological counter-statement. Both are logical men of science and aim to rest on facts. Why do they define and value the “psychical” differently?

To explain the phenomena in the world of our experience, to answer the question “why?” rather than only the question “what?”, is one of the foremost objectives of all rational inquiry; and especially, scientific research in its various branches strives to go beyond a mere description of its subject matter by providing an explanation of the phenomena it investigates. While there is rather general agreement about this chief objective of science, there exists considerable difference of opinion as to the function and the essential characteristics of scientific explanation. In the present essay, an attempt will be made to shed some light on these issues by means of an elementary survey of the basic pattern of scientific explanation and a subsequent more rigorous analysis of the concept of law and of the logical structure of explanatory arguments.