Aristotle, Prior Analytics. Book I, Chapter 08

1. Since, however, to exist, to exist from necessity, and to exist contingently, are different; (for many things exist, indeed, yet not from necessity, but other things neither necessarily exist, nor, in short, exists, yet may happen to exist), it is evident, that there will be a different syllogism of each of these, and from terms not having a similitude of subsistence: but one syllogism will consist of necessary terms; another of such as have an existence; and another of such as are contingent.

2. In necessary syllogisms, therefore, the like will nearly take place, as in those which simply exist; for the terms being similarly posited in simply existing, and in existing or not existing from necessity, there will be, and there will not be a syllogism; except that they differ in the existing or not existing from necessity, being added to the terms.

3. For a privative assertion is in a similar manner converted, And we similarly assign to be in the whole of a thing, and to be predicated of every. In other things, therefore, it is demonstrated after the same manner through conversion, that the conclusion is necessary, just as in existing or being present with a thing. But in the middle figure when the universal proposition is affirmative, and the particular proposition privative; and again in the third figure, when the universal is categoric, but the particular proposition privative, there will not would be demonstration; but it is necessary, Something being proposed with which one of the extremes is not present, to make a syllogism of this; for of this there will be a necessary conclusion. If, however, a necessary conclusion is proposed of the proposed term, a necessary conclusion of some individual of that term will also be produced; For the thing proposed is a part of it. But each of the syllogisms will be found in its proper figure.

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