The Complete Aristotle (eng.)

assumed that A is a possible attribute for all
B. It is necessary then that A is possible for all C. For though
the assumption we made is false and not impossible, the conclusion
is impossible. It is possible also in the first figure to bring
about the impossibility, by assuming that B belongs to C. For if B
belongs to all C, and A is possible for all B, then A would be
possible for all C. But the assumption was made that A is not
possible for all C.
    We must understand ‘that which belongs to all’ with no
limitation in respect of time, e.g. to the present or to a
particular period, but simply without qualification. For it is by
the help of such premisses that we make syllogisms, since if the
premiss is understood with reference to the present moment, there
cannot be a syllogism. For nothing perhaps prevents ‘man’ belonging
at a particular time to everything that is moving, i.e. if nothing
else were moving: but ‘moving’ is possible for every horse; yet
‘man’ is possible for no horse. Further let the major term be
‘animal’, the middle ‘moving’, the the minor ‘man’. The premisses
then will be as before, but the conclusion necessary, not possible.
For man is necessarily animal. It is clear then that the universal
must be understood simply, without limitation in respect of
time.
    Again let the premiss AB be universal and negative, and assume
that A belongs to no B, but B possibly belongs to all C. These
propositions being laid down, it is necessary that A possibly
belongs to no C. Suppose that it cannot belong, and that B belongs
to C, as above. It is necessary then that A belongs to some B: for
we have a syllogism in the third figure: but this is impossible.
Thus it will be possible for A to belong to no C; for if at is
supposed false, the consequence is an impossible one. This
syllogism then does not establish that which is possible according
to the definition, but that which does not necessarily belong to
any part of the subject (for this is the contradictory of the
assumption which was made: for it was supposed that A necessarily
belongs to some C, but the syllogism per impossibile establishes
the contradictory which is opposed to this). Further, it is clear
also from an example that the conclusion will not establish
possibility. Let A be ‘raven’, B ‘intelligent’, and C ‘man’. A then
belongs to no B: for no intelligent thing is a raven. But B is
possible for all C: for every man may possibly be intelligent. But
A necessarily belongs to no C: so the conclusion does not establish
possibility. But neither is it always necessary. Let A be ‘moving’,
B ‘science’, C ‘man’. A then will belong to no B; but B is possible
for all C. And the conclusion will not be necessary. For it is not
necessary that no man should move; rather it is not necessary that
any man should move. Clearly then the conclusion establishes that
one term does not necessarily belong to any instance of another
term. But we must take our terms better.
    If the minor premiss is negative and indicates possibility, from
the actual premisses taken there can be no syllogism, but if the
problematic premiss is converted, a syllogism will be possible, as
before. Let A belong to all B, and let B possibly belong to no C.
If the terms are arranged thus, nothing necessarily follows: but if
the proposition BC is converted and it is assumed that B is
possible for all C, a syllogism results as before: for the terms
are in the same relative positions. Likewise if both the relations
are negative, if the major premiss states that A does not belong to
B, and the minor premiss indicates that B may possibly belong to no
C. Through the premisses actually taken nothing necessary results
in any way; but if the problematic premiss is converted, we shall
have a syllogism. Suppose that A belongs to no B, and B may
possibly belong to no C. Through these comes nothing necessary. But
if B is assumed to be possible for all C (and this is true) and if
the premiss AB remains as before, we shall again have the same
syllogism. But if it be assumed that B does not belong to any C,
instead of possibly not belonging, there cannot be a syllogism
anyhow, whether the premiss AB is negative or affirmative. As
common instances of a necessary and positive relation we may take
the terms white-animal-snow: of a necessary and negative relation,
white-animal-pitch. Clearly then if the terms are universal, and
one of the premisses is assertoric, the other