The most divergent views have prevailed among philosophers both as to what a category is or signifies, and as to what or how many the really ultimate categories are. Is a category, such as substance, or quality, or quantity, a mode of real being revealed to the knowing mind, as most ancient and medieval philosophers thought, with Aristotle and St. Thomas? or is it a mental mode imposed on reality by the knowing mind, as many modern philosophers have thought, with Kant and after him? It is for the Theory of Knowledge to examine this alternative; nor shall we discuss it here except very incidentally: for we shall assume as true the broad affirmative answer to the first alternative. That is to say, we shall hold that the mind is able to see, in the categories generally, modes of reality; rejecting the sceptical conclusions of Kantism in regard to the power of the Speculative Reason, and the principles which lead to such conclusions. As to the number and classification of the ultimate categories, this is obviously a question which cannot be settled _a priori_ by any such purely deductive analysis of the concept of being as Hegel seems to have attempted; but only _a posteriori_, _i.e._ by an analysis of experience in its broadest sense as including Matter and Spirit, Nature and Mind, Object and Subject of Thought, and even the Process of Thought itself. Moreover it is not surprising that with the progress of philosophical reflection, certain categories should have been studied more deeply at certain epochs than ever previously, that they should have been “discovered” so to speak, not of course in the sense that the human mind had not been previously in possession of them, but in the sense that because of closer study they furnished the mind with a richer and fuller power of “explaining” things. It is natural, too, that historians of philosophy, intent on tracing the movement of philosophic thought, should be inclined to over-emphasize the _relativity_ of the categories, as regards their “explaining” value—their relativity to the general mentality of a certain epoch or period.(216) But there is danger here of confounding certain large _hypothetical conceptions_, which are found to yield valuable results at a certain stage in the progress of the sciences,(217) with the categories proper of real being. If the mind of man is of the same nature in all men, if it contemplates the same universe, if it is capable of reaching truth about this universe—real truth which is immutable,—then the modes of being which it apprehends in the universe, and by conceiving which it interprets the latter, must be in the universe as known, and must be there immutably. Nowhere do we find this more clearly illustrated than in the futility of the numerous attempts of modern philosophers to deny the reality of the category of _substance_, and to give an intelligible interpretation of experience without the aid of this category. We shall see that as a matter