occur in knowledge of perception (with which alone we are here concerned) are time, space, and causality. The whole of pure mathematics and pure natural science _a priori_ is based entirely upon these. Therefore it is only in these sciences that knowledge finds no obscurity, does not rest upon what is incomprehensible (groundless, _i.e._, will), upon what cannot be further deduced. It is on this account that Kant wanted, as we have said, to apply the name science specially and even exclusively to these branches of knowledge together with logic. But, on the other hand, these branches of knowledge show us nothing more than mere connections, relations of one idea to another, form devoid of all content. All content which they receive, every phenomenon which fills these forms, contains something which is no longer completely knowable in its whole nature, something which can no longer be entirely explained through something else, something then which is groundless, through which consequently the knowledge loses its evidence and ceases to be completely lucid. This that withholds itself from investigation, however, is the thing-in-itself, is that which is essentially not idea, not object of knowledge, but has only become knowable by entering that form. The form is originally foreign to it, and the thing-in-itself can never become entirely one with it, can never be referred to mere form, and, since this form is the principle of sufficient reason, can never be completely explained. If therefore all mathematics affords us an exhaustive knowledge of that which in the phenomena is quantity, position, number, in a word, spatial and temporal relations; if all etiology gives us a complete account of the regular conditions under which phenomena, with all their determinations, appear in time and space, but, with it all, teaches us nothing more than why in each case this particular phenomenon must appear just at this time here, and at this place now; it is clear that with their assistance we can never penetrate to the inner nature of things. There always remains something which no explanation can venture to attack, but which it always presupposes; the forces of nature, the definite mode of operation of things, the quality and character of every phenomenon, that which is without ground, that which does not depend upon the form of the phenomenal, the principle of sufficient reason, but is something to which this form in itself is foreign, something which has yet entered this form, and now appears according to its law, a law, however, which only determines the appearance, not that which appears, only the how, not the what, only the form, not the content. Mechanics, physics, and chemistry teach the rules and laws according to which the forces of impenetrability, gravitation, rigidity, fluidity, cohesion, elasticity, heat, light, affinity, magnetism, electricity, &c., operate; that is to say, the law,