It has been maintained that the law of cause and effect could never be proved; it is either an innate idea prior to experience (Schopenhauer and Schopenhauer’s interpretation of Kant) or it is an assumption derived from experience of which (since experience is not as yet exhausted) we cannot know whether it will hold good forever (J. S. Mill). In contradistinction to these views we maintain, that the law of cause and effect can satisfactorily be proved to anyone who accepts the principle of identity. So far as the principle of identity is recognised, all the formal laws are unequivocally determined, or popularly expressed they are as they are, they will remain so and cannot be otherwise, they are necessary. All the determining factors and also the procedure of an operation are set forth, no unforeseen events are possible, for the non-formal elements are excluded, and therefore the result will be in one case just as it is in any other. Thus it can _a priori_ be stated that formal laws will always hold good. The question has been raised: Whether or not our knowledge of the apriority of formal laws is independent of experience. We answer: In a certain sense it is dependent upon experience, in another it is not. Historically and evolutionarily it is dependent upon experience. A store of innumerable experiences has to be gained before a rational creature will be able to make the abstraction of pure forms. As soon as this stage is attained, man possesses a world in himself. He can now experiment within himself with mental images, for instance with numbers: he can calculate. His mental operations with pure form are called “pure thought” and “pure thought” is now opposed to “experience through the senses.” The word “experience” accordingly is used in two ways; it has a broad and a narrow meaning. In its broad sense it means any acquisition of knowledge generally, in its narrow sense it means knowledge acquired through the sense-activity alone. Our knowledge of the apriority of formal laws rests upon experience in its broad sense, but not upon experience in its narrow sense. We can never derive the idea of necessity from sense-impressions. John Stuart Mill in rejecting innate ideas saw no other way than to derive the formal laws from experience (taking here experience in the broad sense). Yet he did not make a distinction between formal thought and sense-experience. He considered the nature of sense-experience as typical for all experience. And thus, again, taking experience in the narrow sense of the term, he was from his premises perfectly justified in rejecting the idea of necessity. If the process of cause and effect is really a synthesis of two things represented in two different sense-impressions following each other, then indeed we have no guarantee that the same sequences will always be observed; and there may be worlds in which the law of causality does not operate. Mill saw