
Prior Analytics  Book II
previously considered the particular question. For when he thinks that
the mule is with foal he has not the knowledge in the sense of its
actual exercise, nor on the other hand has his thought caused an error
contrary to his knowledge: for the error contrary to the knowledge
of the universal would be a syllogism.
But he who thinks the essence of good is the essence of bad will
think the same thing to be the essence of good and the essence of bad.
Let A stand for the essence of good and B for the essence of bad,
and again C for the essence of good. Since then he thinks B and C
identical, he will think that C is B, and similarly that B is A,
consequently that C is A. For just as we saw that if B is true of
all of which C is true, and A is true of all of which B is true, A
is true of C, similarly with the word 'think'. Similarly also with the
word 'is'; for we saw that if C is the same as B, and B as A, C is the
same as A. Similarly therefore with 'opine'. Perhaps then this is
necessary if a man will grant the first point. But presumably that
is false, that any one could suppose the essence of good to be the
essence of bad, save incidentally. For it is possible to think this in
many different ways. But we must consider this matter better.
22
Whenever the extremes are convertible it is necessary that the
middle should be convertible with both. For if A belongs to C
through B, then if A and C are convertible and C belongs everything to
which A belongs, B is convertible with A, and B belongs to
everything to which A belongs, through C as middle, and C is
convertible with B through A as middle. Similarly if the conclusion is
negative, e.g. if B belongs to C, but A does not belong to B,
neither will A belong to C. If then B is convertible with A, C will be
convertible with A. Suppose B does not belong to A; neither then
will C: for ex hypothesi B belonged to all C. And if C is
