derived from experience, "by means of the causal principle existing a priori in our intellect." Kant held, on the contrary, that our idea of space is not a mere conception, but a pure intuition; that it belongs to the faculty of sense; that it is in fact one of the necessary conditions of sensation, without which experience itself is impossible; and, finally, that the "causal principle" originates in the higher faculty of the understanding, to which sense delivers up its presentations ready cast in the mold of space. In the philosophy of Kant, therefore, the "causal principle" has nothing to do with the origin of the idea of space, which emerges before sensation rises to the sphere of that principle. Kant, among his endless subtilties, distinguished indeed between the form of the intuition and the formal intuition, in which space is represented as an object, and with which unity of representation is given through the understanding; but the determination of this unity, he taught, is contained a priori in the intuition, not developed a posteriori, gradually or otherwise. As a formal intuition, space, like every other sensuous intuition, Kant maintained, is subject to the categories of the understanding, causality included, but the subjection, besides extending to all sensuous intuitions alike, is a priori, and as such incapable of expansion or contraction by experience, the possibility of which presupposes it. In the Kantian theory causality has nothing special to do in any mode with the idea of space in any aspect. In common with the other categories, it is simply presupposed in every intuition of sense, that of space with the rest. It lies at the bottom of the possibility of experience in general. Moreover, an a priori principle, as already intimated, is not a germ susceptible of growth, but rather a die, which for ever impresses the same form. A new kind of impression necessitates a new kind of die, and, if you would have a new kind of determination, you must get a new a priori principle; a given principle can not be altered to suit emergencies. Experience may sharpen but not remodel it: as well expect the metal to remodel the die that cuts it. An a priori principle reconstructed a posteriori is an article which the author of the "Critique of Pure Reason" happened never to turn out from his workshop, although, if it had found entrance, he infallibly would have turned it out in double quick time.
Yet Professor Zöllner thinks not only that the "causal principle" is a special agent in producing our conception of space, but that "this, in particular, is to be said of the three dimensions of our present conception of space," implying thereby, in the teeth of the Kantian dogma, that we have arrived by degrees at a conception of three dimensions, halting for a time at two, if not halting before at one. Indeed, Professor Crookes, expounding in "The Quarterly Journal of Science" Professor Zöllner's theory, affirms as much explicitly. "The totality of all empirical experience," he states, doubtless repeating the language of his theorist, "is communicated to the intellect by the senses, i.e.,