such a connexion (i.e., causation), I cannot exhibit it, like the concepts of mathematics, by (Anschauung) visualising them, a priori, and so show its possibility a priori. This concept, together with the principles of its application, always requires, if it shall hold a priori—as is requisite in metaphysics—a justification and deduction of its possibility, because we cannot otherwise know how far it holds good, and whether it can be used in experience only or beyond it also. Therefore in metaphysics, as a speculative science of pure reason, we can never appeal to common sense, but may do so only when we are forced to surrender it, and to renounce all purely speculative cognition, which must always be knowledge, and consequently when we forego metaphysics itself and its instruction, for the sake of adopting a rational faith which alone may be possible for us, and sufficient to our wants, perhaps even more salutary than knowledge itself. For in this case the attitude of the question is quite altered. Metaphysics must be science, not only as a whole, but in all its parts, otherwise it is nothing; because, as a speculation of pure reason, it finds a hold only on general opinions. Beyond its field, however, probability and common sense may be used with advantage and justly, but on quite special principles, of which the importance always depends on the reference to practical life. This is what I hold myself justified in requiring for the possibility of metaphysics as a science. APPENDIX. ON WHAT CAN BE DONE TO MAKE METAPHYSICS ACTUAL AS A SCIENCE. Since all the ways heretofore taken have failed to attain the goal, and since without a preceding critique of pure reason it is not likely ever to be attained, the present essay now before the public has a fair title to an accurate and careful investigation, except it be thought more advisable to give up all pretensions to metaphysics, to which, if men but would consistently adhere to their purpose, no objection can be made.