Those who speak of the philosophy of science do not all have the same sort of study in mind. For some it is speculation about the overall nature of the world. Others take it to be basic theory of knowledge and perception. And for still others, it is a branch of philosophical analysis focused specifically on science. The present book is meant to be a study falling under this last category. Generally, such a study has two aspects: one, methodological, dealing with the logical structure of science, the other, substantive, dealing with scientific concepts. Our concern here is primarily methodological; and, where discussion veers at times towards substantive matters, this will be largely for the purpose of illustrating underlying methodological points. It should also be added that our considerations will be of a general sort, intended to apply to all of science with no special concern for any particular divisions. Except in an incidental manner, therefore, we shall give no primary attention to special problems in the methodology of the social sciences or in the philosophy of physics or of biology. And if we draw the larger portion of our examples from the physical rather than from the behavioral sciences, this is done merely for simplicity, succinctness, and similar conveniences of exposition rather than out of specialized concern for any particular area.