Thus much in reply to him who complains of the doctrine here stated as, 1st, Too narrow, or, 2d, As insufficiently established. But, 3d, in reply to him who wishes to see it further pursued or applied, I say that the possible applications are perhaps infinite. With respect to those made by Kant himself, they are chiefly contained in his main and elementary work, the _Critik der reinen Vernunft_; and they are of a nature to make any man melancholy. Indeed, let a man consider merely this one notion of _causation_; let him reflect on its origin; let him remember that, agreeably to this origin, it follows that we have no right to view anything _in rerum naturâ_ as objectively, or in itself, a cause; that, when, upon the fullest philosophic proof, we call A the cause of B, we do in fact only subsume A under the notion of a cause--we invest it with that function under that relation; that the whole proceeding is merely with respect to a _human_ understanding, and by way of indispensable _nexus_ to the several parts of our experience; finally, that there is the greatest reason to doubt whether the idea of _causation_ is at all applicable to any other world than this, or any other than a human experience. Let a man meditate but a little on this or other aspects of this transcendental philosophy, and he will find the steadfast earth itself rocking as it were beneath his feet; a world about him which is in some sense a world of deception; and a world before him which seems to promise a world of confusion, or "_a world not realised_." All this he might deduce for himself without further aid from Kant. However, the particular purposes to which Kant applies his philosophy, from the difficulties which beset them, are unfitted for anything below a regular treatise. Suffice it to say here, that, difficult as these speculations are from one or two embarrassing doctrines on the Transcendental Consciousness, and depressing as they are from their general tendency, they are yet painfully irritating to the curiosity, and especially so from a sort of _experimentum crucis_ which they yield in the progress of their development on behalf of the entire doctrine of Kant--a test which, up to this hour, has offered defiance to any hostile hand. The test or defiance which I speak of takes the shape of certain _antinomies_ (so they are termed), severe adamantine arguments, affirmative and negative, on two or three celebrated problems, with no appeal to any possible decision, but one which involves the Kantian doctrines. A _quæstio vexata_ is proposed--for instance, the _infinite divisibility of matter_; each side of this question, _thesis_ and _antithesis_, is argued; the logic is irresistible, the links are perfect, and for each side alternately there is a verdict, thus terminating in the most triumphant _reductio ad absurdum_,--viz. that A, at one and the same time and in the same sense, is and is not B,--from which no escape is available but through a Kantian solution.