Again, in fact it is
false to assume that that which is an attribute of all B is an
attribute of no A, for if it be an attribute of all B, it must be an
attribute of some A. If then C is nevertheless assumed to be an
attribute of all B but of no A, C-B will be true but C-A false.
It is thus clear that in the case of atomic propositions erroneous
inference will be possible not only when both premisses are false
but also when only one is false.
17
In the case of attributes not atomically connected with or
disconnected from their subjects, (a) (i) as long as the false
conclusion is inferred through the 'appropriate' middle, only the
major and not both premisses can be false. By 'appropriate middle' I
mean the middle term through which the contradictory-i.e. the
true-conclusion is inferrible. Thus, let A be attributable to B
through a middle term C: then, since to produce a conclusion the
premiss C-B must be taken affirmatively, it is clear that this premiss
must always be true, for its quality is not changed. But the major A-C
is false, for it is by a change in the quality of A-C that the
conclusion becomes its contradictory-i.
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