Philosophy is distinguished from other departments of human knowledge and inquiry by the kind of questions it raises and its method of solving them, rather than by its subject-matter. There are no facts which do not come within the province of philosophy, no matter to what particular science they belong. It is the business of science to describe the data accurately, to classify them, and to formulate the general laws of their occurrence. But in order to do this science must use language; and the exact use of the language requires the definition of terms; and terms can only be defined by careful reflective analysis of the experience which gives them meaning and by critical comparison with each other. Science begins with the concepts and terms developed by unreflective common sense and refines their meaning as it proceeds. This reflective analysis and critical comparison of its fundamental concepts is the philosophy of science. It is not a seeking to know more, but a seeking to know better, to know more clearly, what is known already. To perform this philosophical part of his task the scientist has to step beyond the boundaries of his own particular science, for many of his concepts are common to several sciences. Further, even the concepts peculiar to his own science must not be defined in such a way that they have implications which conflict with those of other sciences. Where such conflict arises one or other is wrong and each must re-examine his concepts.