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the terms the supposable and the opinable in preference to the
phrase suggested。
40
Since the expressions 'pleasure is good' and 'pleasure is the
good' are not identical； we must not set out the terms in the same
way； but if the syllogism is to prove that pleasure is the good； the
term must be 'the good'； but if the object is to prove that pleasure
is good； the term will be 'good'。 Similarly in all other cases。
41
It is not the same； either in fact or in speech； that A belongs to
all of that to which B belongs； and that A belongs to all of that to
all of which B belongs： for nothing prevents B from belonging to C；
though not to all C： e。g。 let B stand for beautiful； and C for
white。 If beauty belongs to something white； it is true to say that
beauty belongs to that which is white； but not perhaps to everything
that is white。 If then A belongs to B； but not to everything of
which B is predicated； then whether B belongs to all C or merely
belongs to C； it is not necessary that A should belong； I do not say
to all C； but even to C at all。 But if A belongs to everything of
which B is truly stated； it will follow that A can be said of all of
that of all of which B is said。 If however A is said of that of all of
which B may be said； nothing prevents B belonging to C； and yet A
not belonging to all C or to any C at all。 If then we take three terms
it is clear that the expression 'A is said of all of which B is
said' means this； 'A is said of all the things of which B is said'。
And if B is said of all of a third term； so also is A： but if B is not
said of all of the third term； there is no necessity that A should
be said of all of it。
We must not suppose that something absurd results through setting
out the terms： for we do not use the existence of this particular
thing； but imitate the geometrician who says that 'this line a foot
long' or 'this straight line' or 'this line without breadth' exists
although it does not； but does not use the diagrams in the sense
that he reasons from them。 For in general； if two things are not
related as whole to part and part to whole； the prover does not
prove from them； and so no syllogism a is formed。 We （I mean the
learner） use the process of setting out terms like perception by
sense； not as though it were impossible to demonstrate without these
illustrative terms； as it is to demonstrate without the premisses of
the syllogism。
42
We should not forget that in the same syllogism not all
conclusions are reached through one figure； but one through one
figure； another through another。 Clearly then we must analyse
arguments in accordance with this。 Since not every problem is proved
in every figure； but certain problems in each figure； it is clear from
the conclusion in what figure the premisses should be sought。
43
In reference to those arguments aiming at a definition which have
been directed to prove some part of the definition； we must take as
a term the point to which the argument has been directed； not the
whole definition： for so we shall be less likely to be disturbed by
the length of the term： e。g。 if a man proves that water is a drinkable
liquid； we must take as terms drinkable and water。
44
Further we must not try to reduce hypothetical syllogisms； for
with the given premisses it is not possible to reduce them。 For they
have not been proved by syllogism； but assented to by agreement。 For
instance if a man should suppose that unless there is one faculty of
contraries； there cannot be one science； and should then argue that
not every faculty is of contraries； e。g。 of what is healthy and what
is sickly： for the same thing will then be at the same time healthy
and sickly。 He has shown that there is not one faculty of all
contraries； but he has not proved that there is not a science。 And yet
one must agree。 But the agreement does not come from a syllogism；
but from an hypothesis。 This argument cannot be reduced： but the proof
that there is not a single faculty can。 The latter argument perhaps
was a syllogism： but the former was an hypothesis。
The same holds good of arguments which are brought to a conclusion
per impossibile。 These cannot be analysed either； but the reduction to
what is impossible can be analysed since it is proved by syllogism；
though the rest of the argument cannot； because the conclusion is
reached from an hypothesis。 But these differ from the previous
arguments： for in the former a preliminary agreement must be reached
if one is to accept the conclusion； e。g。 an agreement that if there is
proved to be one faculty of contraries； then contraries fall under the
same science； whereas in the latter； even if no preliminary
agreement has been made； men still accept the reasoning； because the
falsity is patent； e。g。 the falsity of what follows from the
assumption that the diagonal is commensurate； viz。 that then odd
numbers are equal to evens。
Many other arguments are brought to a conclusion by the help of an
hypothesis； these we ought to consider and mark out clearly。 We
shall describe in the sequel their differences； and the various ways
in which hypothetical arguments are formed： but at present this much
must be clear； that it is not possible to resolve such arguments
into the figures。 And we have explained the reason。
45
Whatever problems are proved in more than one figure； if they have
been established in one figure by syllogism； can be reduced to another
figure； e。g。 a negative syllogism in the first figure can be reduced
to the second； and a syllogism in the middle figure to the first；
not all however but some only。 The point will be clear in the
sequel。 If A belongs to no B； and B to all C； then A belongs to no
C。 Thus the first figure； but if the negative statement is
converted； we shall have the middle figure。 For B belongs to no A； and
to all C。 Similarly if the syllogism is not universal but
particular； e。g。 if A belongs to no B； and B to some C。 Convert the
negative statement and you will have the middle figure。
The universal syllogisms in the second figure can be reduced to
the first； but only one of the two particular syllogisms。 Let A belong
to no B and to all C。 Convert the negative statement； and you will
have the first figure。 For B will belong to no A and A to all C。