He who has recognized the form of the principle of sufficient reason, which appears in pure time as such, and on which all counting and calculating are based, has thereby also recognized the whole essence of time. It is nothing more than that very form of the principle of sufficient reason, and it has no other quality or attribute. Succession is the form of the principle of sufficient reason in time, and succession is the whole essence and nature of time. Further, he who has recognized the principle of sufficient reason as it rules in mere, purely perceived space, has thereby exhausted the whole nature of space. For this is absolutely nothing else but the possibility of the reciprocal determinations of its parts by one another, which is called position. The detailed consideration of this, and the formulation of the results flowing from it into abstract conceptions for convenient application, form the subject-matter of the whole of geometry. Now in just the same way, he who has recognized that form of the principle of sufficient reason which governs the content of those forms (of time and space), their perceptibility, i.e., matter, and hence the law of causality, has thereby recognized the entire essence and nature of matter as such; for matter is absolutely nothing but causality, as anyone sees immediately the moment he reflects on it. Thus its being is its acting; it is not possible to conceive for it any other being. Only as something acting does it fill space and time; its action on the immediate object (which is itself matter) conditions the perception in which alone it exists. The consequence of the action of every material object on another is known only in so far as the latter now acts on the immediate object in a way different from that in which it acted previously; it consists in this alone. Thus cause and effect are the whole essence and nature of matter; its being is its acting. (Details of this are to be found in the essay On the Principle of Sufficient Reason, § 21, p. 77.) The substance of everything material is therefore very appropriately called in German Wirklichkeit,14 a word much more expressive than Realität. That on which it acts, again, is always matter; thus its whole being and essence consist only in the orderly and regular change produced by one part of it in another; consequently, its being and essence are entirely relative, according to a relation that is valid only within its limits, and hence just like time and space.
Time and space, however, each by itself, can be represented in intuition even without matter; but matter cannot be so represented without time and space. The form inseparable from it presupposes space, and its action, in which its entire existence consists, always concerns a change, and hence a determination of time. But time and space are not only, each by itself, presupposed by matter, but a combination of the two constitutes its essential nature, just because this, as we have shown, consists in action, in causality. All the innumerable phenomena and conditions of things that can be conceived could thus lie side by side in endless space without limiting one another, or even follow one another in endless time without disturbing one another. Thus a necessary relation of these phenomena to one another, and a rule determining them according to this relation, would then not be at all needful, or even applicable. Thus, in the case of all juxtaposition in space and of all change in time, so long as each of these two forms by itself, and without any connexion with the other, had its course and duration, there would be no causality at all, and as this constitutes the real essence of matter, there would also be no matter. But the law of causality receives its meaning and necessity only from the fact that the essence of change does not consist in the mere variation of states or conditions in themselves. On the contrary, it consists in the fact that, at the same place in space, there is now one condition or state and then another, and at one and the same point of time there is here this state and there that state. Only this mutual limitation of time and space by each other gives meaning, and at the same time necessity, to a rule according to which change must take place. What is determined by the law of causality is therefore not the succession of states in mere time, but that succession in respect of a particular space, and not only the existence of states at a particular place, but at this place at a particular time. Thus change, i.e., variation occurring according to the causal law, always concerns a particular part of space and a particular part of time, simultaneously and in union. Consequently, causality unites space and time. But we found that the whole essence of matter consists in action, and hence in causality; consequently, space and time must also be united in this, in other words, matter must carry within itself simultaneously the properties and qualities of time and those of space, however much the two are opposed to each other. It must unite within itself what is impossible in each of those two independently, the unstable flight of time with the rigid unchangeable persistence of space; from both it has infinite divisibility. Accordingly, through it we find coexistence first brought about. This could not be either in mere time, that knows no juxtaposition, or in mere space, that knows no before, after, or now. But the coexistence of many states constitutes in fact the essence of reality, for through it permanence or duration first becomes possible. Permanence is knowable only in the change of that which exists simultaneously with what is permanent; but also only by means of what is permanent in variation does variation receive the character of change, i.e., of the alteration of quality and form in spite of the persistence of substance, i.e., of matter.15 In mere space, the world would be rigid and immovable, with no succession, no change, no action; but with action arises also the representation of matter. Again, in mere time everything would be fleeting, with no persistence, no juxtaposition, and therefore no coexistence, consequently no permanence or duration, and thus also once more no matter. Only through the combination of time and space arises matter, that is to say, the possibility of coexistence, and so of duration; and again, through duration the possibility of persistence of substance with change of states and conditions.16 As matter has its essential nature in the union of time and space, it bears in all respects the stamp of both. It shows its origin from space partly through the form that is inseparable from it, and particularly through its persistence (substance), (since variation belongs to time alone, but in it alone and for it nothing is permanent). The a priori certainty of persistence or substance is therefore to be wholly and entirely derived from that of space.17 Matter reveals its origin from time in quality (accident), without which it never appears, and which is positively always causality, action on other matter, and hence change (a concept of time). The conformity to law of this action, however, always has reference to space and time simultaneously, and only thus has meaning. The legislative force of causality relates solely and entirely to the determination as to what kind of state or condition must appear at this time and in this place. On this derivation of the basic determinations of matter from the forms of our knowledge, of which we are a priori conscious, rests our knowledge a priori of the sure and certain properties of matter. These are space-occupation, i.e., impenetrability, i.e., effectiveness, then extension, infinite divisibility, persistence, i.e., indestructibility, and finally mobility. On the other hand, gravity, notwithstanding its universality, is to be attributed to knowledge a posteriori, although Kant in his Metaphysical Rudiments of Natural Science (p. 71: Rosenkranz’s edition, p. 372) asserts that it is knowable a priori.
But as the object in general exists only for the subject as the representation thereof, so does every special class of representations exist only for an equally special disposition in the subject, which is called a faculty of knowledge. The subjective correlative of time and space in themselves, as empty forms, was called by Kant pure sensibility, and this expression may be retained, as Kant was the pioneer here, although it is not quite suitable; for sensibility presupposes matter. The subjective correlative of matter or of causality, for the two are one and the same, is the understanding, and it is nothing more than this. To know causality is the sole function of the understanding, its only power, and it is a great power embracing much, manifold in its application, and yet unmistakable in its identity throughout all its manifestations. Conversely, all causality, hence all matter, and consequently the whole of reality, is only for the understanding, through the understanding, in the understanding. The first, simplest, ever-present manifestation of understanding is perception of the actual world. This is in every way knowledge of the cause from the effect, and therefore all perception is intellectual. Yet one could never arrive at perception, if some effect were not immediately known, and thus served as the starting-point. But this is the action or effect on animal bodies. To this extent these bodies are the immediate objects of the subject; through them the perception of all other objects is brought about. The changes experienced by every animal body are immediately known, that is to say, felt; and as this effect is referred at once to its cause, there arises the perception of the latter as an object. This relation is no conclusion in abstract concepts, it does not happen through reflection, it is not arbitrary, but is immediate, necessary, and certain. It is the cognitive method of the pure understanding, without which perception would never be attained; there would remain only a dull, plant-like consciousness of the changes of the immediate object which followed one another in a wholly meaningless way, except in so far as they might have a meaning for the will either as pain or pleasure. But as with the appearance of the sun the visible world makes its appearance, so at one stroke does the understanding through its one simple function convert the dull meaningless sensation into perception. What the eye, the ear, or the hand experiences is not perception; it is mere data. Only by the passing of the understanding from the effect to the cause does the world stand out as perception extended in space, varying in respect of form, persisting through all time as regards matter. For the understanding unites space and time in the representation of matter, that is to say, of effectiveness. This world as representation exists only through the understanding, and also only for the understanding. In the first chapter of my essay On Vision and Colours, I have explained how the understanding produces perception out of the data furnished by the senses; how by comparing the impressions received by the different senses from the same object the child learns perception; how this alone throws light on so many phenomena of the senses, on single vision with two eyes, on double vision in the case of squinting, or in the case where we look simultaneously at objects that lie behind one another at unequal distances, and on every illusion 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 my essay On the Principle of Sufficient Reason (§21). All that is said there has its necessary place here, and therefore ought really to be said again. But as I am almost as reluctant 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 that essay instead of repeating it, and here assume that it is known.
The process by which children, and persons who are born blind and have been operated on, learn to see; single vision of whatever is perceived with two eyes; double vision and double touch, occurring when the organs of sense are displaced from their usual position; the upright appearance of objects, whereas their image in the eye is inverted; the attributing of colour to external objects, whereas it is merely an inner function, a division, through polarization, of the activity of the eye; and finally also the stereoscope; all these are solid and irrefutable proofs that all perception is not only of the senses, but of the intellect; in other words, pure knowledge through the understanding of the cause from the effect. Consequently, it presupposes the law of causality, and on the knowledge of this depends all perception, and therefore all experience, by virtue of its primary and entire possibility. The converse, namely that knowledge of the causal law results from experience, is not the case; this was the scepticism of Hume, and is first refuted by what is here said. For the independence of the knowledge of causality from all experience, in other words, its a priori character, can alone be demonstrated from the dependence of all experience on it. Again, this can be done only by proving, in the manner here indicated, and explained in the passages above referred to, that the knowledge of causality is already contained in perception generally, in the domain of which all experience is to be found, and hence that it exists wholly a priori in respect of experience, that it does not presuppose experience, but is presupposed thereby as a condition. But this cannot be demonstrated in the manner attempted by Kant, which I criticize in the essay On the Principle of Sufficient Reason (§ 23).