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_ 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.