By making the categories the peculiar possession of the mind, we attribute to these cognitions the essential characteristic of being anterior to sensation, or, as it is also termed, of existing _a priori_: we are taught that not only are they not derived from experience, nor taught us by observation, but further that they are presupposed by all observation, for they set up, in scholastic jargon, the conditions which make experience possible. They represent the personal contribution of the mind to the knowledge of nature, and, consequently, to admit them is to admit that the mind is not, in the presence of the world, reduced to the passive state of a _tabula rasa_, and that the faculties of the mind are not a transformation of sensation. Only these categories do not supplement sensation, they do not obviate it, nor allow it to be conjectured beforehand. They remain empty forms so long as they are not applied to experience; they are the rules of cognition and not the objects of cognition, the means of knowing and not the things known; they render knowledge possible, but do not of themselves constitute it, Experience through the senses still remains a necessary condition to the knowledge of the external world. It may be said that the senses give the matter of knowledge, and that the categories of the understanding give the form of it. Matter cannot exist without form, nor form without matter; it is the union of the two which produces cognition. Such is the simplest idea that can be given of the Kantian theory of categories, or, if it is preferred to employ the term often used and much discussed, such is the theory of the Kantian idealism, This theory, I will say frankly, hardly harmonises with the ideas I have set forth up to this point. To begin with, let us scrutinise the relation which can exist between the subject and the object. We have seen that the existence of the subject is hardly admissible, for it could only be an object in disguise. Cognition is composed in reality of an object and an act of consciousness. Now, how can we know if this act of consciousness, by adding itself to the object, modifies it and causes it to appear other than it is?