The Complete Aristotle (eng.)

considers the two propositions together. So it is
evident that if he knows the one and does not know the other, he
will fall into error. And this is the relation of knowledge of the
universal to knowledge of the particular. For we know no sensible
thing, once it has passed beyond the range of our senses, even if
we happen to have perceived it, except by means of the universal
and the possession of the knowledge which is proper to the
particular, but without the actual exercise of that knowledge. For
to know is used in three senses: it may mean either to have
knowledge of the universal or to have knowledge proper to the
matter in hand or to exercise such knowledge: consequently three
kinds of error also are possible. Nothing then prevents a man both
knowing and being mistaken about the same thing, provided that his
knowledge and his error are not contrary. And this happens also to
the man whose knowledge is limited to each of the premisses and who
has not 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.
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    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
convertible with B, B is convertible also with A, for C is said of
that of all of which B is said. And if C is convertible in relation
to A and to B, B also is convertible in relation to A. For C
belongs to that to which B belongs: but C does not belong to that
to which A belongs. And this alone starts from the conclusion; the
preceding moods do not do so as in the affirmative syllogism. Again
if A and B are convertible, and similarly C and D, and if A or C
must belong to anything whatever, then B and D will be such that
one or other belongs to anything whatever. For since B belongs to
that to which A belongs, and D belongs to that to which C belongs,
and since A or C belongs to everything, but not together, it is
clear that B or D belongs to everything, but not together. For
example if that which is uncreated is incorruptible and that which
is incorruptible is uncreated, it is necessary that what is created
should be corruptible and what is corruptible should have been
created. For two syllogisms have been put together. Again if A or B
belongs to everything and if C or D belongs to everything, but they
cannot belong together, then when A and C are convertible B and D
are convertible. For if B does not belong to something to which D
belongs, it is clear that A belongs to it. But if A then C: for
they are convertible. Therefore C and D belong together. But this
is impossible. When A belongs to the whole of B and to C and is
affirmed of nothing else, and B also belongs to all C, it is
necessary that A and B should be convertible: for since A is said
of B and C only, and B is affirmed both