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Prior Analytics - Book II   

affirmative, the other negative: consequently, if what is laid down is

contrary to the conclusion, a refutation must take place: for a

refutation is a syllogism which establishes the contradictory. But

if nothing is conceded, a refutation is impossible: for no syllogism

is possible (as we saw) when all the terms are negative: therefore

no refutation is possible. For if a refutation were possible, a

syllogism must be possible; although if a syllogism is possible it

does not follow that a refutation is possible. Similarly refutation is

not possible if nothing is conceded universally: since the fields of

refutation and syllogism are defined in the same way.


It sometimes happens that just as we are deceived in the arrangement

of the terms, so error may arise in our thought about them, e.g. if it

is possible that the same predicate should belong to more than one

subject immediately, but although knowing the one, a man may forget

the other and think the opposite true. Suppose that A belongs to B and

to C in virtue of their nature, and that B and C belong to all D in

the same way. If then a man thinks that A belongs to all B, and B to

D, but A to no C, and C to all D, he will both know and not know the

same thing in respect of the same thing. Again if a man were to make a

mistake about the members of a single series; e.g. suppose A belongs

to B, B to C, and C to D, but some one thinks that A belongs to all B,

but to no C: he will both know that A belongs to D, and think that

it does not. Does he then maintain after this simply that what he

knows, he does not think? For he knows in a way that A belongs to C

through B, since the part is included in the whole; so that what he

knows in a way, this he maintains he does not think at all: but that

is impossible.

In the former case, where the middle term does not belong to the

same series, it is not possible to think both the premisses with

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