through its persistence (substance), the a priori certainty of which is therefore wholly deducible from that of space9 (for variation belongs to time alone, but in it alone and for itself nothing is persistent). Matter shows that it springs from time by quality (accidents), without which it never exists, and which is plainly always causality, action upon other matter, and therefore change (a time concept). The law of this action, however, always depends upon space and time together, and only thus obtains meaning. The regulative function of causality is confined entirely to the determination of what must occupy this time and this space. The fact that we know a priori the unalterable characteristics of matter, depends upon this derivation of its essential nature from the forms of our knowledge of which we are conscious a priori. These unalterable characteristics are space-occupation, i.e., impenetrability, i.e., causal action, consequently, extension, infinite divisibility, persistence, i.e., indestructibility, and lastly mobility: weight, on the other hand, notwithstanding its universality, must be attributed to a posteriori knowledge, although Kant, in his “Metaphysical Introduction to Natural Philosophy,” p. 71 (p. 372 of Rosenkranz's edition), treats it as knowable a priori.
But as the object in general is only for the subject, as its idea, so every special class of ideas is only for an equally special quality in the subject, which is called a faculty of perception. This subjective correlative of time and space in themselves as empty forms, has been named by Kant pure sensibility; and we may retain this expression, as Kant was the first to treat of the subject, though it is not exact, for sensibility presupposes matter. The subjective correlative of matter or of causation, for these two are the same, is understanding, which is nothing more than this. To know causality is its one function, its only power; and it is a great one, embracing much, of manifold application, yet of unmistakable identity in all its manifestations. Conversely all causation, that is to say, all matter, or the whole of reality, is only for the understanding, through the understanding, and in the understanding. The first, simplest, and ever-present example of understanding is the perception of the actual world. This is throughout knowledge of the cause from the effect, and therefore all perception is intellectual. The understanding could never arrive at this perception, however, if some effect did not become known immediately, and thus serve as a starting-point. But this is the affection of the animal body. So far, then, the animal body is the immediate object of the subject; the perception of all other objects becomes possible through it. The changes which every animal body experiences, are immediately known, that is, felt; and as these effects are at once referred to their causes, the perception of the latter as objects arises. This relation is no conclusion in abstract conceptions; it does not arise from reflection, nor is it arbitrary, but immediate, necessary, and certain. It is the method of knowing of the pure understanding, without which there could be no perception; there would only remain a dull plant-like consciousness of the changes of the immediate object, which would succeed each other in an utterly unmeaning way, except in so far as they might have a meaning for the will either as pain or pleasure. But as with the rising of the sun the visible world appears, so at one stroke, the understanding, by means of its one simple function, changes the dull, meaningless sensation into perception. What the eye, the ear, or the hand feels, is not perception; it is merely its data. By the understanding passing from the effect to the cause, the world first appears as perception extended in space, varying in respect of form, persistent through all time in respect of matter; for the understanding unites space and time in the idea of matter, that is, causal action. As the world as idea exists only through the understanding, so also it exists only for the understanding. In the first chapter of my essay on “Light and Colour,” I have already explained how the understanding constructs perceptions out of the data supplied by the senses; how by comparison of the impressions which the various senses receive from the object, a child arrives at perceptions; how this alone affords the solution of so many phenomena of the senses; the single vision of two eyes, the double vision in the case of a squint, or when we try to look at once at objects which lie at unequal distances behind each other; and all illusion which is produced by a sudden alteration in the organs of sense. But I have treated this important subject much more fully and thoroughly in the second edition of the essay on “The Principle of Sufficient Reason,” § 21. All that is said there would find its proper place here, and would therefore have to be said again; but as I have almost as much disinclination to quote myself as to quote others, and as I am unable to explain the subject better than it is explained there, I refer the reader to it, instead of quoting it, and take for granted that it is known.
The process by which children, and persons born blind who have been operated upon, learn to see, the single vision of the double sensation of two eyes, the double vision and double touch which occur when the organs of sense have been displaced from their usual position, the upright appearance of objects while the picture on the retina is upside down, the attributing of colour to the outward objects, whereas it is merely an inner function, a division through polarisation, of the activity of the eye, and lastly the stereoscope—all these are sure and incontrovertible evidence that perception is not merely of the senses, but intellectual—that is, pure knowledge through the understanding of the cause from the effect, and that, consequently, it presupposes the law of causality, in a knowledge of which all perception—that is to say all experience, by virtue of its primary and only possibility, depends. The contrary doctrine that the law of causality results from experience, which was the scepticism of Hume, is first refuted by this. For the independence of the knowledge of causality of all experience—that is, its a priori character—can only be deduced from the dependence of all experience upon it; and this deduction can only be accomplished by proving, in the manner here indicated, and explained in the passages referred to above, that the knowledge of causality is included in perception in general, to which all experience belongs, and therefore in respect of experience is completely a priori, does not presuppose it, but is presupposed by it as a condition. This, however, cannot be deduced in the manner attempted by Kant, which I have criticised in the essay on “The Principle of Sufficient Reason,” § 23.
§ 5. It is needful to guard against the grave error of supposing that because perception arises through the knowledge of causality, the relation of subject and object is that of cause and effect. For this relation subsists only between the immediate object and objects known indirectly, thus always between objects alone. It is this false supposition that has given rise to the foolish controversy about the reality of the outer world; a controversy in which dogmatism and scepticism oppose each other, and the former appears, now as realism, now as idealism. Realism treats the object as cause, and the subject as its effect. The idealism of Fichte reduces the object to the effect of the subject. Since however, and this cannot be too much emphasised, there is absolutely no relation according to the principle of sufficient reason between subject and object, neither of these views could be proved, and therefore scepticism attacked them both with success. Now, just as the law of causality precedes perception and experience as their condition, and therefore cannot (as Hume thought) be derived from them, so object and subject precede all knowledge, and hence the principle of sufficient reason in general, as its first condition; for this principle is merely the form of all objects, the whole nature and possibility of their existence as phenomena: but the object always presupposes the subject; and therefore between these two there can be no relation of reason and consequent. My essay on the principle of sufficient reason accomplishes just this: it explains the content of that principle as the essential form of every object—that is to say, as the universal nature of all objective existence, as something which pertains to the object as such; but the object as such always presupposes the subject as its necessary correlative; and therefore the subject remains always outside the province in which the principle of sufficient reason is valid. The controversy as to the reality of the outer world rests upon this false extension of the validity of the principle of sufficient reason to the subject also, and starting with this mistake it can never understand itself. On the one side realistic dogmatism, looking upon the idea as the effect of the object, desires to separate these two, idea and object, which are really one, and to assume a cause quite different from the idea, an object in itself, independent of the subject, a thing which is quite inconceivable; for even as object it presupposes subject, and so remains its idea. Opposed to this doctrine is scepticism, which makes the same false presupposition that in the idea we have only the effect, never the cause, therefore never real being; that we always know merely the action of the object. But this object, it supposes, may perhaps have no resemblance