On this new ground the difficulty that confronts critical idealism is the same as that affecting the notion of substance. It lies in defending the position against empiricism, from which are borrowed all the arguments against the reality of the cause, while attempting, nevertheless, to preserve the notion of it. How succeed in retaining as an _à priori_ law what empiricism declares to be only an acquired habit? How explain a law of mind imposing a determined order on external phenomena? How can the entirely subjective need of relation determine phenomena to produce themselves in the order desired by our intelligence? The thunder rolls: my mind, in virtue of an innate law, insists on this phenomenon being connected with a certain totality of antecedent phenomena--namely, heat, the formation of clouds charged with electricity of different kinds, the meeting of these clouds, and the combination of the two electricities, &c. How and why have these phenomena produced themselves in order to satisfy my mind? Our author somewhere reproaches the partisans of innate ideas with supposing ideas on one side and phenomena on the other. How can he exonerate Kant's system from this objection? No philosopher ever insisted more than he on the opposition between matter and form, the former being, as he says, "given _à posteriori_," the latter ready prepared _à priori_ in the mind. No philosopher, not even Leibnitz, has more radically separated sensibility which is passive from the understanding whose principle is spontaneity. How do these two opposite principles happen to agree? Even were it pointed out that our senses themselves are innate, since our sensations are but the manifestation of the specific activity of each one of them--light, of the optic nerve, sound, of the acoustic--it still remains certain that our sensations are only subjective as regards their content and not as regards their origin; they arise in virtue of causes to us unknown. How should understanding, by aid of a purely mental law, and in order to its own satisfaction, evoke sensible phenomena from nothingness, and if it had such a power, it could only be in virtue of an active force, that is, of a veritable causality? You say that you require relation, without which there could be no knowledge. And why must there be knowledge because you feel the need of it? And why should there not be in the understanding a need of unity and relation that sensibility does not satisfy? To say that the mind at the same time that it thinks the law produces phenomena conformable to that law, is to make the mind itself the cause in the objective and metaphysical sense of the word--is no other than that universal spiritualism that the author began by refuting. We are therefore very far from admitting his criticism of the principles of causality. Let us go on to the notion of the absolute.