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_. [159] Compare "Die Welt a. W. u. V.," vol. i. p. 551 _et seq._ of the 2nd edition (i. p. 582 _et seq._ of 3rd edition) as to "immaterial substance," and § 52 of the present work as to "reason in general." (Editor's note.) There exists accordingly a _fourfold_ necessity, in conformity with the _four_ forms of the Principle of Sufficient Reason:-- 1^o. _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. 2^o. _Physical necessity_, according to the law of causality, in virtue of which, as soon as the cause presents itself, the effect must infallibly follow. 3^o. _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.