The Complete Aristotle (eng.)

since the
syllogism also is necessary. But that which is necessary is wider
than the syllogism: for every syllogism is necessary, but not
everything which is necessary is a syllogism. Consequently, though
something results when certain propositions are assumed, we must
not try to reduce it directly, but must first state the two
premisses, then divide them into their terms. We must take that
term as middle which is stated in both the remisses: for it is
necessary that the middle should be found in both premisses in all
the figures.
    If then the middle term is a predicate and a subject of
predication, or if it is a predicate, and something else is denied
of it, we shall have the first figure: if it both is a predicate
and is denied of something, the middle figure: if other things are
predicated of it, or one is denied, the other predicated, the last
figure. For it was thus that we found the middle term placed in
each figure. It is placed similarly too if the premisses are not
universal: for the middle term is determined in the same way.
Clearly then, if the same term is not stated more than once in the
course of an argument, a syllogism cannot be made: for a middle
term has not been taken. Since we know what sort of thesis is
established in each figure, and in which the universal, in what
sort the particular is described, clearly we must not look for all
the figures, but for that which is appropriate to the thesis in
hand. If the thesis is established in more figures than one, we
shall recognize the figure by the position of the middle term.
33
    Men are frequently deceived about syllogisms because the
inference is necessary, as has been said above; sometimes they are
deceived by the similarity in the positing of the terms; and this
ought not to escape our notice. E.g. if A is stated of B, and B of
C: it would seem that a syllogism is possible since the terms stand
thus: but nothing necessary results, nor does a syllogism. Let A
represent the term ‘being eternal’, B ‘Aristomenes as an object of
thought’, C ‘Aristomenes’. It is true then that A belongs to B. For
Aristomenes as an object of thought is eternal. But B also belongs
to C: for Aristomenes is Aristomenes as an object of thought. But A
does not belong to C: for Aristomenes is perishable. For no
syllogism was made although the terms stood thus: that required
that the premiss AB should be stated universally. But this is
false, that every Aristomenes who is an object of thought is
eternal, since Aristomenes is perishable. Again let C stand for
‘Miccalus’, B for ‘musical Miccalus’, A for ‘perishing to-morrow’.
It is true to predicate B of C: for Miccalus is musical Miccalus.
Also A can be predicated of B: for musical Miccalus might perish
to-morrow. But to state A of C is false at any rate. This argument
then is identical with the former; for it is not true universally
that musical Miccalus perishes to-morrow: but unless this is
assumed, no syllogism (as we have shown) is possible.
    This deception then arises through ignoring a small distinction.
For if we accept the conclusion as though it made no difference
whether we said ‘This belong to that’ or ‘This belongs to all of
that’.
34
    Men will frequently fall into fallacies through not setting out
the terms of the premiss well, e.g. suppose A to be health, B
disease, C man. It is true to say that A cannot belong to any B
(for health belongs to no disease) and again that B belongs to
every C (for every man is capable of disease). It would seem to
follow that health cannot belong to any man. The reason for this is
that the terms are not set out well in the statement, since if the
things which are in the conditions are substituted, no syllogism
can be made, e.g. if ‘healthy’ is substituted for ‘health’ and
‘diseased’ for ‘disease’. For it is not true to say that being
healthy cannot belong to one who is diseased. But unless this is
assumed no conclusion results, save in respect of possibility: but
such a conclusion is not impossible: for it is possible that health
should belong to no man. Again the fallacy may occur in a similar
way in the middle figure: ‘it is not possible that health should
belong to any disease, but it is possible that health should belong
to every man, consequently it is not possible that disease should
belong to any man’. In the third figure the fallacy results in
reference to possibility. For health and diseae and knowledge and
ignorance, and in