An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

IMMEDIATE INFERENCE. 8 I
the attempted converse is not even true as a matter of
fact. Similarly, from
"
some candidates who sit for an
"
we cannot infer that
examination do not pass it,"
some
candidates who pass an examination do not sit for it."
From the four examples just given, we derive the
second rule of conversion, no term must be distributed
in the converse which was not known to be distrib
uted in the convertend.
There are some further aspects of the process of
conversion which must not escape the student s atten
tion. In converting I and E we change neither quan
tity nor quality ; the converse of SiP is PiS, and of SeP
is PeS. This is called simple conversion, to distinguish
it from the process which is necessary in converting A.
Here, though we do not change the quality, we change
the quantity ; the converse of SaP is PiS. This is called
conversion by limitation, the equivalent of the Aristo
telian phrase avrio-rpocf)}] Kara /Jbepo^ (An. Prior., i. 2).
The mediaeval logicians called it conversio per acridens.
The conversion of an A proposition without limit
ation is a frequent source of fallacy. From
"
ill-doers
are ill-dreaders
"
slip into the unlimited converse,
(understood as universal) it is easy to
"
ill-dreaders arc ill-
doers," also understood universally. Similarly, "all
beautiful things are agreeable
not follow that
"
may be true, but it does
"
all agreeable things are beautiful. We
may know from the matter of the proposition that S and
P are coextensive. But the single proposition
"
all S is
" P does not logically express the relation of coincidence
or coextension between S and P ; to do this, we require
the two propositions together,
( (a) All S is P.
\ (b)
All P is S.
The logical converse is thus to be distinguished from
F