Elsewhere Kant asserts, that a representation only shows reality (which, I conclude, means that it is distinguished from a mere mental image) by our recognising its necessary connection with other representations subject to rule (the causal law) and its place in a determined order of the time-relations of our representations. But of how few representations are we able to know the place assigned to them by the law of causality in the chain of causes and effects! Yet we are never embarrassed to distinguish objective from subjective representations: real, from imaginary objects. When asleep, we are unable to make this distinction, for our brain is then isolated from the peripherical nervous system, and thereby from external influences. In our dreams therefore, we take imaginary for real things, and it is only when we awaken: that is, when our nervous sensibility, and through this the outer world, once more comes within our consciousness, that we become aware of our mistake; still, even in our dreams, so long as they last, the causal law holds good, only an impossible material is often substituted for the usual one. We might almost think that Kant was influenced by Leibnitz in writing the passage we have quoted, however much he differs from him in all the rest of his philosophy; especially if we consider that Leibnitz expresses precisely similar views, when, for instance, he says: "La vérité des choses sensibles ne consiste que dans la liaison des phénomènes, qui doit avoir sa raison, et c'est ce qui les distingue des songes. —— Le vrai Critérion, en matière des objets des sens, est la liaison des phénomènes, qui garantit les vérités de fait, à l'egard des choses sensibles hors de nous."[101]
It is clear that in proving the à priority and the necessity of the causal law by the fact that the objective succession of changes is known to us only by means of that law, and that, in so far, causality is a condition for all experience, Kant fell into a very singular error, and one which is indeed so palpable, that the only way we can account for it is, by supposing him to have become so absorbed in the à priori part of our knowledge, that he lost sight of what would have been evident to anyone else. The only correct demonstration of the à priority of the causal law is given by me in § 21 of the present work. That à priority finds its confirmation every moment in the infallible security with which we expect experience to tally with the causal law: that is to say, in the apodeictic certainty we ascribe to it, a certainty which differs from every other founded on induction—the certainty, for instance, of empirically known laws of Nature—in that we can conceive no exception to the causal law anywhere within the world of experience. We can, for instance, conceive that in an exceptional case the law of gravitation might cease to act, but not that this could happen without a cause.
Kant and Hume have fallen into opposite errors in their proofs. Hume asserts that all consequence is mere sequence; whereas Kant affirms that all sequence must necessarily be consequence. Pure Understanding, it is true, can only conceive consequence (causal result), and is no more able to conceive mere sequence than to conceive the difference between right and left, which, like sequence, is only to be grasped by means of pure Sensibility. Empirical knowledge of the following of events in Time is, indeed, just as possible as empirical knowledge of juxtaposition of things in Space (this Kant denies elsewhere), but the way in which things follow upon one another in general in Time can no more be explained, than the way in which one thing follows from another (as the effect of a cause): the former knowledge is given and conditioned by pure Sensibility; the latter, by pure Understanding. But in asserting that knowledge of the objective succession of phenomena can only be attained by means of the causal law, Kant commits the same error with which he reproaches Leibnitz:[102] that of "intellectualising the forms of Sensibility."—My view of succession is the following one. We derive our knowledge of the bare possibility of succession from the form of Time, which belongs to pure Sensibility. The succession of real objects, whose form is precisely Time, we know empirically, consequently as actual. But it is through the Understanding alone, by means of Causality, that we gain knowledge of the necessity of a succession of two states: that is, of a change; and even the fact that we are able to conceive the necessity of a succession at all, proves already that the causal law is not known to us empirically, but given us à priori. The Principle of Sufficient Reason is the general expression for the fundamental form of the necessary connection between all our objects, i.e. representations, which lies in the innermost depths of our cognitive faculty: it is the form common to all representations, and the only source of the conception of necessity, which contains absolutely nothing else in it and no other import, than that of the following of the consequence, when its reason has been established. Now, the reason why this principle determines the order of succession in Time in the class of representations we are now investigating, in which it figures as the law of causality, is, that Time is the form of these representations, therefore the necessary connection appears here as the rule of succession. In other forms of the principle of sufficient reason, the necessary connection it always demands will appear under quite different forms from that of Time, therefore not as succession; still it always retains the character of a necessary connection, by which the identity of the principle under all its forms, or rather the unity of the root of all the laws of which that principle is the common expression, reveals itself.
If Kant's assertion were correct, which I dispute, our only way of knowing the reality of succession would be through its necessity; but this would presuppose an Understanding that embraced all the series of causes and effects at once, consequently an omniscient Understanding. Kant has burdened the Understanding with an impossibility, merely in order to have less need of Sensibility.
How can we reconcile Kant's assertion that our only means of knowing the objective reality of succession is by the necessity with which effect follows cause, with his other assertion[103] that succession in Time is our only empirical criterion for determining which of two states is cause, and which effect. Who does not see the most obvious circle here?
If we knew objectiveness of succession through Causality, we should never be able to think it otherwise than as Causality, and then it would be nothing else than Causality. For, if it were anything else, it would have other distinctive signs by which to be recognised; now this is just what Kant denies. Accordingly, if Kant were right, we could not say: "This state is the effect of that one, wherefore it follows it;" for following and being an effect, would be one and the same thing, and this proposition a tautology. Besides, if we do away with all distinction between following upon and following from, we once more yield the point to Hume, who declared all consequence to be mere sequence and therefore denied that distinction likewise.
Kant's proof would, consequently, be reduced to this: that, empirically, we only know actuality of succession; but as besides we recognise necessity of succession in certain series of occurrences, and even know before all experience that every possible occurrence must have a fixed place in some one of these series, the reality and the à priority of the causal law follow as a matter of course, the only correct proof of the latter being the one I have given in § 21 of this work.
Parallel with the Kantian theory: that the causal nexus alone renders objective succession and our knowledge of it possible, there runs another: that coexistence and our knowledge of it are only possible through reciprocity. In the "Critique of Pure Reason" they are presented under the title: "Third Analogy of Experience." Here Kant goes so far as to say that "the co-existence of phenomena, which exercise no reciprocal action on one another, but are separated by a perfectly empty space, could never become an object of possible perception"[104] (which, by the way, would be a proof à priori that there is no empty space between the fixed stars), and that "the light which plays between our eyes and celestial bodies"—an expression conveying surreptitiously the thought, that this starlight not only acts upon our eyes, but is acted upon by them also—"produces an intercommunity between us and them, and proves the co-existence of the latter." Now, even empirically, this last assertion is false; since the sight of a fixed star by no means proves its coexistence simultaneously with its spectator, but, at most, its existence some years, nay even some centuries before. Besides, this second Kantian theory stands and falls with the first, only it is far more easily detected; and the nullity of the whole conception of reciprocity has been shown in § 20.
The arguments I have brought forward against Kant's proof may be compared with two previous attacks made on it by Feder,[105] and by G. E. Schulze.[106]
Not without considerable hesitation did I thus venture (in 1813) to attack a theory which had been universally received as a demonstrated truth, is repeated even now in the latest publications,[107] and forms a chief point in the doctrine of one for whose profound wisdom I have the greatest reverence and admiration; one to whom, indeed, I owe so much, that his spirit might truly say to me, in the words of Homer: