The first writer who explicitly separated the two chief significations of our principle, and stated the difference between them in detail, was therefore Wolf. Wolf, however, does not place the principle of sufficient reason in Logic, as is now the custom, but in Ontology. True, in § 71 he urges the necessity of not confounding the principle of sufficient reason of knowing with that of cause and effect; still he does not clearly determine here wherein the difference consists. Indeed, he himself mistakes the one for the other; for he quotes instances of cause and effect in confirmation of the _principium rationis sufficientis_ in this very chapter, _de ratione sufficiente_, §§ 70, 74, 75, 77, which, had he really wished to preserve that distinction, ought rather to have been quoted in the chapter _de causis_ of the same work. In said chapter he again brings forward precisely similar instances, and once more enunciates the _principium cognoscendi_ (§ 876), which does not certainly belong to it, having been already discussed, yet which serves to introduce the immediately following clear and definite distinction between this principle and the law of causality, §§ 881-884. _Principium_, he continues, _dicitur id, quod in se continet rationem alterius_; and he distinguishes _three_ kinds: 1. PRINCIPIUM FIENDI (_causa_), which he defines as _ratio actualitatis alterius_, e.g., _si lapis calescit, ignis aut radii solares sunt rationes, cur calor lapidi insit_.--2. PRINCIPIUM ESSENDI, which he defines as _ratio possibilitatis alterius; in eodem, exemplo, ratio possibilitatis, cur lapis calorem recipere possit, est in essentia seu modo compositionis lapidis_. This last conception seems to me inadmissible. If it has any meaning at all, possibility means correspondence with the general conditions of experience known to us _à priori_, as Kant has sufficiently shown. From these conditions we know, with respect to Wolf's instance of the stone, that changes are possible as effects proceeding from causes: we know, that is, that one state can succeed another, if the former contains the conditions for the latter. In this case we find, as effect, the state of being warm in the stone; as cause, the preceding state of a limited capacity for warmth in the stone and its contact with free heat. Now, Wolf's naming the first mentioned property of this state _principium essendi_, and the second, _principium fiendi_, rests upon a delusion caused by the fact that, so far as the stone is concerned, the conditions are more lasting and can therefore wait longer for the others. That the stone should be as it is: that is, that it should be chemically so constituted as to bring with it a particular degree of specific heat, consequently a capacity for heat which stands in inverse proportion to its specific heat; that besides it should, on the other hand, come into contact with free heat, is the consequence of a whole chain of antecedent causes, all of