The Complete Aristotle (eng.)

belongs
to all E, and A to all C, consequently A belongs to all E. If C and
G are identical, A must belong to some of the Es: for A follows C,
and E follows all G. If F and D are identical, A will belong to
none of the Es by a prosyllogism: for since the negative
proposition is convertible, and F is identical with D, A will
belong to none of the Fs, but F belongs to all E. Again, if B and H
are identical, A will belong to none of the Es: for B will belong
to all A, but to no E: for it was assumed to be identical with H,
and H belonged to none of the Es. If D and G are identical, A will
not belong to some of the Es: for it will not belong to G, because
it does not belong to D: but G falls under E: consequently A will
not belong to some of the Es. If B is identical with G, there will
be a converted syllogism: for E will belong to all A since B
belongs to A and E to B (for B was found to be identical with G):
but that A should belong to all E is not necessary, but it must
belong to some E because it is possible to convert the universal
statement into a particular.
    It is clear then that in every proposition which requires proof
we must look to the aforesaid relations of the subject and
predicate in question: for all syllogisms proceed through these.
But if we are seeking consequents and antecedents we must look for
those which are primary and most universal, e.g. in reference to E
we must look to KF rather than to F alone, and in reference to A we
must look to KC rather than to C alone. For if A belongs to KF, it
belongs both to F and to E: but if it does not follow KF, it may
yet follow F. Similarly we must consider the antecedents of A
itself: for if a term follows the primary antecedents, it will
follow those also which are subordinate, but if it does not follow
the former, it may yet follow the latter.
    It is clear too that the inquiry proceeds through the three
terms and the two premisses, and that all the syllogisms proceed
through the aforesaid figures. For it is proved that A belongs to
all E, whenever an identical term is found among the Cs and Fs.
This will be the middle term; A and E will be the extremes. So the
first figure is formed. And A will belong to some E, whenever C and
G are apprehended to be the same. This is the last figure: for G
becomes the middle term. And A will belong to no E, when D and F
are identical. Thus we have both the first figure and the middle
figure; the first, because A belongs to no F, since the negative
statement is convertible, and F belongs to all E: the middle figure
because D belongs to no A, and to all E. And A will not belong to
some E, whenever D and G are identical. This is the last figure:
for A will belong to no G, and E will belong to all G. Clearly then
all syllogisms proceed through the aforesaid figures, and we must
not select consequents of all the terms, because no syllogism is
produced from them. For (as we saw) it is not possible at all to
establish a proposition from consequents, and it is not possible to
refute by means of a consequent of both the terms in question: for
the middle term must belong to the one, and not belong to the
other.
    It is clear too that other methods of inquiry by selection of
middle terms are useless to produce a syllogism, e.g. if the
consequents of the terms in question are identical, or if the
antecedents of A are identical with those attributes which cannot
possibly belong to E, or if those attributes are identical which
cannot belong to either term: for no syllogism is produced by means
of these. For if the consequents are identical, e.g. B and F, we
have the middle figure with both premisses affirmative: if the
antecedents of A are identical with attributes which cannot belong
to E, e.g. C with H, we have the first figure with its minor
premiss negative. If attributes which cannot belong to either term
are identical, e.g. C and H, both premisses are negative, either in
the first or in the middle figure. But no syllogism is possible in
this way.
    It is evident too that we must find out which terms in this
inquiry are identical, not which are different or contrary, first
because the object of our investigation is the middle term, and the
middle term must be not diverse but identical. Secondly, wherever
it happens that a syllogism results from taking contraries or terms
which cannot belong to the same thing, all arguments can be reduced
to the aforesaid moods, e.g. if B and F are contraries or cannot
belong to the same thing. For if