In § 20 it has already been shown, that the law of causality does not admit of reciprocity, since the effect never can be the cause of its cause; therefore the conception of reciprocity is, in its right sense, inadmissible. Reciprocity, according to the Principle of Sufficient Reason of knowing, would only be possible between equivalent conceptions, since the spheres of these alone cover each other mutually. Apart from these, it only gives rise to a vicious circle.
§ 49. Necessity.
The Principle of Sufficient Reason in all its forms is the sole principle and the sole support of all necessity. For necessity has no other true and distinct meaning than that of the infallibility of the consequence when the reason is posited. Accordingly every necessity is conditioned: absolute, i.e., unconditioned, necessity therefore is a contradicto in adjecto. For to be necessary can never mean anything but to result from a given reason. By defining it as "what cannot not be," on the other hand, we give a mere verbal definition, and screen ourselves behind an extremely abstract conception to avoid giving a definition of the thing. But it is not difficult to drive us from this refuge by inquiring how the non-existence of anything can be possible or even conceivable, since all existence is only given empirically. It then comes out, that it is only possible so far as some reason or other is posited or present, from which it follows. To be necessary and to follow from a given reason, are thus convertible conceptions, and may always, as such, be substituted one for the other. The conception of an "ABSOLUTELY necessary Being" which finds so much favour with pseudo-philosophers, contains therefore a contradiction: it annuls by the predicate "absolute" (i.e., "unconditioned by anything else") the only determination which makes the "necessary" conceivable. Here again we have an instance of the improper use of abstract conceptions to play off a metaphysical artifice such as those I have already pointed out in the conceptions "immaterial substance," "cause in general," "absolute reason," &c. &c.[159] I can never insist too much upon all abstract conceptions being checked by perception.
There exists accordingly a fourfold necessity, in conformity with the four forms of the Principle of Sufficient Reason:—
1o. Logical necessity, according to the principle of sufficient reason of knowing, in virtue of which, when once we have admitted the premisses, we must absolutely admit the conclusion.
2o. Physical necessity, according to the law of causality, in virtue of which, as soon as the cause presents itself, the effect must infallibly follow.
3o. Mathematical necessity, according to the principle of sufficient reason of being, in virtue of which, every relation which is stated in a true geometrical theorem, is as that theorem affirms it to be, and every correct calculation remains irrefutable.
4o. Moral necessity, in virtue of which, every human being, every animal even, is compelled, as soon as a motive presents itself, to do that which alone is in accordance with the inborn and immutable character of the individual. This action now follows its cause therefore as infallibly as every other effect, though it is less easy here to predict what that effect will be than in other cases, because of the difficulty we have in fathoming and completely knowing the individual empirical character and its allotted sphere of knowledge, which is indeed a very different thing from ascertaining the chemical properties of a neutral salt and predicting its reaction. I must repeat this again and again on account of the dunces and blockheads who, in defiance of the unanimous authority of so many great thinkers, still persist in audaciously maintaining the contrary, for the benefit of their old woman's philosophy. I am not a professor of philosophy, forsooth, that I need bow to the folly of others.
§ 50. Series of Reasons and Consequences.
According to the law of causality, the condition is itself always conditioned, and, moreover, conditioned in the same way; therefore, there arises a series in infinitum a parte ante. It is just the same with the Reason of Being in Space: each relative space is a figure; it has its limits, by which it is connected with another relative space, and which themselves condition the figure of this other, and so on throughout all dimensions in infinitum. But when we examine a single figure in itself, the series of reasons of being has an end, because we start from a given relation, just as the series of causes comes to an end if we stop at pleasure at any particular cause. In Time, the series of reasons of being has infinite extension both a parte ante, and a parte post, since each moment is conditioned by a preceding one, and necessarily gives rise to the following. Time has therefore neither beginning nor end. On the other hand, the series of reasons of knowledge—that is, a series of judgments, each of which gives logical truth to the other—always ends somewhere, i.e., either in an empirical, a transcendental, or a metalogical truth. If the reason of the major to which we have been led is an empirical truth, and we still continue asking why, it is no longer a reason of knowledge that is asked for, but a cause—in other words, the series of reasons of knowing passes over into the series of reasons of becoming. But if we do the contrary, that is, if we allow the series of reasons of becoming to pass over into the series of reasons of knowing, in order to bring it to an end, this is never brought about by the nature of the thing, but always by a special purpose: it is therefore a trick, and this is the sophism known by the name of the Ontological Proof. For when a cause, at which it seems desirable to stop short in order to make it the first cause, has been reached by means of the Cosmological Proof, we find out that the law of causality is not so easily brought to a standstill, and still persists in asking why: so it is simply set aside and the principle of sufficient reason of knowing, which from a distance resembles it, is substituted in its stead; and thus a reason of knowledge is given in the place of the cause which had been asked for—a reason of knowledge derived from the conception itself which has to be demonstrated, the reality of which is therefore still problematical: and this reason, as after all it is one, now has to figure as a cause. Of course the conception itself has been previously arranged for this purpose, and reality slightly covered with a few husks just for decency's sake has been placed within it, so as to give the delightful surprise of finding it there—as has been shown in Section 7. On the other hand, if a chain of judgments ultimately rests upon a principle of transcendental or of metalogical truth, and we still continue to ask why, we receive no answer at all, because the question has no meaning, i.e., it does not know what kind of reason it is asking for.