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. For the Principle of Sufficient Reason is the _principle of all explanation: to explain a thing_ means, to reduce its given existence or connection to some form or other of the Principle of Sufficient Reason, in accordance with which form that existence or connection necessarily is that which it is. The Principle of Sufficient Reason itself, _i.e._, the connection expressed by it in any of its forms, cannot therefore be further explained; because there exists no principle by which to explain the source of all explanation: just as the eye is unable to see itself, though it sees everything else. There are of course series of motives, since the resolve to attain an end becomes the motive for the resolve to use a whole series of means; still this series invariably ends _à parte priori_ in a representation belonging to one of our two first classes, in which lies the motive which originally had the power to set this individual will in motion. The fact that it was able to do this, is a datum for knowing the empirical character here given, but it is impossible to answer the question why that particular motive acts upon that particular character; because the intelligible character lies outside Time and never becomes an Object. Therefore the series of motives, as such, finds its termination in some such final motive and, according to the nature of its last link, passes into the series of causes, or that of reasons of knowledge: that is to say, into the former, when that last link is a real object; into the latter, when it is a mere conception. § 51. _Each Science has for its Guiding Thread one of the Forms of the Principle of Sufficient Reason in preference to the others._