Aristotle also denies reciprocity in the strict sense; for he remarks that two things may certainly be reciprocal causes of each other, but only if this is understood in a different sense of each of them; for example, that one acts upon the other as the motive, but the latter acts upon the former as the cause of its movement. We find in two passages the same words: Physic., lib. ii. c. 3, and Metaph., lib. v. c. 2. Εστι δε τινα και αλληλων αιτια; οἱον το πονειν αιτιον της ευεξιας, και αὑτη του πονειν; αλλ᾽ ου τον αυτον τροπον, αλλα το μεν ὡς τελος, το δε ὡς αρχη κινησεως. (Sunt præterea quæ sibi sunt mutuo causæ, ut exercitium bonæ habitudinis, et hæc exercitii: at non eodem modo, sed hæc ut finis, aliud ut principium motus.) If, besides this, he had accepted a reciprocity proper, he would have introduced it here, for in both passages he is concerned with enumerating all the possible kinds of causes. In the Analyt. post., lib. ii. c. 11, he speaks of a circle of causes and effects, but not of reciprocity.
4. The categories of Modality have this advantage over all others, that what is expressed through each of them really corresponds to the form of judgment from which it is derived; which with the other categories is scarcely ever the case, because for the most part they are deduced from the forms of judgment with the most capricious violence.
Thus that it is the conceptions of the possible, the actual, and the necessary which occasion the problematic, assertatory, and apodictic forms of judgment, is perfectly true; but that those conceptions are special, original forms of knowledge of the understanding which cannot be further deduced is not true. On the contrary, they spring from the single original form of all knowledge, which is, therefore, known to us a priori, the principle of sufficient reason; and indeed out of this the knowledge of necessity [pg 067] springs directly. On the other hand, it is only because reflection is applied to this that the conceptions of contingency, possibility, impossibility, and actuality arise. Therefore all these do not by any means spring from one faculty of the mind, the understanding, but arise through the conflict of abstract and intuitive knowledge, as will be seen directly.
I hold that to be necessary and to be the consequent of a given reason are absolutely interchangeable notions, and completely identical. We can never know, nor even think, anything as necessary, except so far as we regard it as the consequent of a given reason; and the conception of necessity contains absolutely nothing more than this dependence, this being established through something else, and this inevitable following from it. Thus it arises and exists simply and solely through the application of the principle of sufficient reason. Therefore, there is, according to the different forms of this principle, a physical necessity (the effect from the cause), a logical (through the ground of knowing, in analytical judgments, syllogisms, &c.), a mathematical (according to the ground of being in time and space), and finally a practical necessity, by which we intend to signify not determination through a pretended categorical imperative, but the necessary occurrence of an action according to the motives presented, in the case of a given empirical character. But everything necessary is only so relatively, that is, under the presupposition of the reason from which it follows; therefore absolute necessity is a contradiction. With regard to the rest, I refer to § 49 of the essay on the principle of sufficient reason.
The contradictory opposite, i.e., the denial of necessity, is contingency. The content of this conception is, therefore, negative—nothing more than this: absence of the connection expressed by the principle of sufficient reason. Consequently the contingent is also always merely relative. It is contingent in relation to something which is [pg 068] not its reason. Every object, of whatever kind it may be—for example, every event in the actual world—is always at once necessary and contingent, necessary in relation to the one condition which is its cause: contingent in relation to everything else. For its contact in time and space with everything else is a mere coincidence without necessary connection: hence also the words chance, συμπτωμα, contingens. Therefore an absolute contingency is just as inconceivable as an absolute necessity. For the former would be simply an object which stood to no other in the relation of consequent to its reason. But the inconceivability of such a thing is just the content of the principle of sufficient reason negatively expressed, and therefore this principle must first be upset before we can think an absolute contingency; and even then it itself would have lost all significance, for the conception of contingency has meaning only in relation to that principle, and signifies that two objects do not stand to each other in the relation of reason and consequent.
In nature, which consists of ideas of perception, everything that happens is necessary; for it proceeds from its cause. If, however, we consider this individual with reference to everything else which is not its cause, we know it as contingent; but this is already an abstract reflection. Now, further, let us abstract entirely from a natural object its causal relation to everything else, thus its necessity and its contingency; then this kind of knowledge comprehends the conception of the actual, in which one only considers the effect, without looking for the cause, in relation to which one would otherwise have to call it necessary, and in relation to everything else contingent. All this rests ultimately upon the fact that the modality of the judgment does not indicate so much the objective nature of things as the relation of our knowledge to them. Since, however, in nature everything proceeds from a cause, everything actual is also necessary, yet only so far as it is at this time, in this place; for only so far does [pg 069] determination by the law of causality extend. Let us leave, however, concrete nature and pass over to abstract thinking; then we can present to ourselves in reflection all the natural laws which are known to us partly a priori, partly only a posteriori, and this abstract idea contains all that is in nature at any time, in any place, but with abstraction from every definite time and place; and just in this way, through such reflection, we have entered the wide kingdom of the possible. But what finds no place even here is the impossible. It is clear that possibility and impossibility exist only for reflection, for abstract knowledge of the reason, not for knowledge of perception; although it is the pure forms of perception which supply the reason with the determination of the possible and impossible. According as the laws of nature, from which we start in the thought of the possible and impossible, are known a priori or a posteriori, is the possibility or impossibility metaphysical or physical.
From this exposition, which requires no proof because it rests directly upon the knowledge of the principle of sufficient reason and upon the development of the conceptions of the necessary, the actual, and the possible, it is sufficiently evident how entirely groundless is Kant's assumption of three special functions of the understanding for these three conceptions, and that here again he has allowed himself to be disturbed by no reflection in the carrying out of his architectonic symmetry.
To this, however, we have to add the other great mistake, that, certainly according to the procedure of earlier philosophy, he has confounded the conceptions of necessity and contingency with each other. That earlier philosophy has applied abstraction to the following mistaken use. It was clear that that of which the reason is given inevitably follows, i.e., cannot not be, and thus necessarily is. But that philosophy held to this last determination alone, and said that is necessary which cannot be otherwise, or the opposite of which is impossible. It left, however, the [pg 070] ground and root of such necessity out of account, overlooked the relativity of all necessity which follows from it, and thereby made the quite unthinkable fiction of an absolute necessity, i.e., of something the existence of which would be as inevitable as the consequent of a reason, but which yet was not the consequent of a reason, and therefore depended upon nothing; an addition which is an absurd petitio, for it conflicts with the principle of sufficient reason. Now, starting from this fiction, it explained, in diametrical opposition to the truth, all that is established by a reason as contingent, because it looked at the relative nature of its necessity and compared this with that entirely imaginary absolute necessity, which is self-contradictory in its conception.[5] Now Kant adheres to this fundamentally perverse definition of the contingent and gives it as explanation. (Critique of Pure Reason, V. p. 289-291, 243. V. 301, 419. V. 447, 486, 488.) He falls indeed into the most evident contradiction with himself upon this point, for on p. 301 he says: “Everything contingent has a cause,” and adds, “That is contingent which might possibly not be.” But whatever has a cause cannot possibly not be: thus it is necessary. For the rest, the source of the whole of this false explanation of the necessary and the contingent is to be found in Aristotle in “De Generatione et Corruptione,” lib. ii. c. 9 et 11, where the necessary is explained as that which cannot possibly not be: there stands in opposition [pg 071] to it that which cannot possibly be, and between these two lies that which can both be and not be,—thus that which comes into being and passes away, and this would then be the contingent. In accordance with what has been said above, it is clear that this explanation, like so many of Aristotle's, has resulted from sticking to abstract conceptions without going back to the concrete and perceptible, in which, however, the source of all abstract conceptions lies, and by which therefore they must always be controlled. “Something which cannot possibly not be” can certainly be thought in the abstract, but if we go with it to the concrete, the real, the perceptible, we find nothing to support the thought, even as possible,—as even merely the asserted consequent of a given reason, whose necessity is yet relative and conditioned.
I take this opportunity of adding a few further remarks on these conceptions of modality. Since all necessity rests upon the principle of sufficient reason, and is on this account relative, all apodictic judgments are originally, and according to their ultimate significance, hypothetical. They become categorical only through the addition of an assertatory minor, thus in the conclusion. If this minor is still undecided, and this indecision is expressed, this gives the problematical judgment.
What in general (as a rule) is apodictic (a law of nature), is in reference to a particular case only problematical, because the condition must actually appear which brings the case under the rule. And conversely, what in the particular as such is necessary (apodictic) (every particular change necessary through the cause), is again in general, and predicated universally, only problematical; because the causes which appear only concern the particular case, and the apodictic, always hypothetical judgment, always expresses merely the general law, not the particular case directly. All this has its ground in the fact that possibility exists only in the province of reflection and for the reason; the actual, in the province of perception and for [pg 072] the understanding; the necessary, for both. Indeed, the distinction between necessary, actual, and possible really exists only in the abstract and according to the conception; in the real world, on the other hand, all three fall into one. For all that happens, happens necessarily, because it happens from causes; but these themselves have again causes, so that the whole of the events of the world, great and small, are a strict concatenation of necessary occurrences. Accordingly everything actual is also necessary, and in the real world there is no difference between actuality and necessity, and in the same way no difference between actuality and possibility; for what has not happened, i.e., has not become actual, was also not possible, because the causes without which it could never appear have not themselves appeared, nor could appear, in the great concatenation of causes; thus it was an impossibility. Every event is therefore either necessary or impossible. All this holds good only of the empirically real world, i.e., the complex of individual things, thus of the whole particular as such. If, on the other hand, we consider things generally, comprehending them in abstracto, necessity, actuality, and possibility are again separated; we then know everything which is in accordance with the a priori laws which belong to our intellect as possible in general; that which corresponds to the empirical laws of nature as possible in this world, even if it has never become actual; thus we distinguish clearly the possible from the actual. The actual is in itself always also necessary, but is only comprehended as such by him who knows its cause; regarded apart from this, it is and is called contingent. This consideration also gives us the key to that contentio περι δυνατων between the Megaric Diodorus and Chrysippus the Stoic which Cicero refers to in his book De Fato. Diodorus says: “Only what becomes actual was possible, and all that is actual is also necessary.” Chrysippus on the other hand says: “Much that is possible never becomes actual; for only the necessary becomes [pg 073] actual.” We may explain this thus: Actuality is the conclusion of a syllogism to which possibility gives the premises. But for this is required not only the major but also the minor; only the two give complete possibility. The major gives a merely theoretical, general possibility in abstracto, but this of itself does not make anything possible, i.e., capable of becoming actual. For this the minor also is needed, which gives the possibility for the particular case, because it brings it under the rule, and thereby it becomes at once actual. For example: