Propositions

Every demonstration will proceed through three terms and no more, unless the same conclusion is established by different pairs of propositions; e.g. the conclusion E may be established through the propositions A and B, and through the propositions C and D, or through the propositions A and B, or A and C, or B and C.

For nothing prevents there being several middles for the same terms. But in that case there is not one but several syllogisms.

Or again when each of the propositions A and B is obtained by syllogistic inference, e.g. by means of D and E, and again B by means of F and G. Or one may be obtained by syllogistic, the other by inductive inference.

But thus also the syllogisms are many; for the conclusions are many, e.g. A and B and C. But if this can be called one syllogism, not many, the same conclusion may be reached by more than three terms in this way, but it cannot be reached as C is established by means of A and B.

Suppose that the proposition E is inferred from the premises A, B, C, and D. It is necessary then that of these one should be related to another as whole to part: for it has already been proved that if a syllogism is formed some of its terms must be related in this way. Suppose then that A stands in this relation to B. Some conclusion then follows from them. It must either be E or one or other of C and D, or something other than these.

1. If it is E the syllogism will have A and B for its sole premises.

But if C and D are so related that one is whole, the other part, some conclusion will follow from them also; and it must be either E, or one or other of the propositions A and B, or something other than these.

If it is

• (i) E, or
• (ii) A or B, either
  • (i) the syllogisms will be more than one, or
  • (ii) the same thing happens to be inferred by means of several terms only in the sense which we saw to be possible.
• But if (iii) the conclusion is other than E or A or B, the syllogisms will be many, and unconnected with one another.

But if C is not so related to D as to make a syllogism, the propositions will have been assumed to no purpose, unless for the sake of induction or of obscuring the argument or something of the sort.

2. But if from the propositions A and B there follows not E but some other conclusion, and if from C and D either A or B follows or something else, then there are several syllogisms, and they do not establish the conclusion proposed: for we assumed that the syllogism proved E.

If no conclusion follows from C and D, it turns out that these propositions have been assumed to no purpose, and the syllogism does not prove the original proposition.

So it is clear that every demonstration and every syllogism will proceed through three terms only.

This being evident, it is clear that a syllogistic conclusion follows from two premises and not from more than two. For the three terms make two premises, unless a new premise is assumed, as was said at the beginning, to perfect the syllogisms.

In whatever syllogistic argument the premises through which the main conclusion follows (for some of the preceding conclusions must be premises) are not even in number, this argument either has not been drawn syllogistically or it has assumed more than was necessary to establish its thesis.

If then syllogisms are taken with respect to their main premises, every syllogism will consist of an even number of premises and an odd number of terms (for the terms exceed the premises by one), and the conclusions will be half the number of the premises.

But whenever a conclusion is reached by means of prosyllogisms or by means of several continuous middle terms, e.g. the proposition AB by means of the middle terms C and D, the number of the terms will similarly exceed that of the premises by one (for the extra term must either be added outside or inserted: but in either case it follows that the relations of predication are one fewer than the terms related), and the premises will be equal in number to the relations of predication.

The premises however will not always be even, the terms odd; but they will alternate-when the premises are even, the terms must be odd; when the terms are even, the premises must be odd: for along with one term one premise is added, if a term is added from any quarter. Consequently since the premises were (as we saw) even, and the terms odd, we must make them alternately even and odd at each addition.

But the conclusions will not follow the same arrangement either in respect to the terms or to the premises. For if one term is added, conclusions will be added less by one than the preexisting terms: for the conclusion is drawn not in relation to the single term last added, but in relation to all the rest, e.g. if to ABC the term D is added, two conclusions are thereby added, one in relation to A, the other in relation to B. Similarly with any further additions. And similarly too if the term is inserted in the middle: for in relation to one term only, a syllogism will not be constructed. Consequently the conclusions will be much more numerous than the terms or the premises.