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

158
MEDIATE INFERENCE
principle, or by bringing it under a higher class : then the
dictum of Aristotle is directly applicable, and the figure is
the first. If the argument aims at a negative conclusion,
separating two things by reasoning
from the fact that an
attribute which is characteristic of one is absent in the
other ; the figure is the second. If the argument aims
at establishing a rule, a general or partly general state
ment by an instance; or if it endeavours to deny such a rule
by means of a negative instance: then the figure is the third.
In order to express the argument strictly in the
form of mood and figure,
it is usually necessary to
make changes in the given expression of it, supplying
any premise which may be understood but not ex
pressed, according to Hamilton s postulate, that what
is implicit in thought may be made explicit in language
(ch. III.
4).
It is a mistake to say, as Jevons does,
that such changes "are of an extra-logical character,
and belong more properly to
"
the science of language ;
for they are changes made in order that the words
may express the true logical relations of the thoughts.
Jevons quotes from the Port Royal Logic two examples
of arguments which, he says, "cannot be proved by the
rules of the syllogism," and yet are perfectly valid. The
examples are : "
(a) The sun is a thing insensible the
;
Persians worship the sun therefore the Persians ; worship
a thing insensible." (b} "The Divine Law commands us
to honour kings Louis XIV. is a ;
Divine Law commands us to honour
king ;
Louis
therefore the
XIV." Now
if we were limited to making merely grammatical changes
in these arguments, it would be difficult if not impossible
to express them as strict syllogisms. But it should have
been evident that they can be so expressed. The first
of them adduces an instance in support of the general
statement that Persians are worshippers of a thing insen
sible, hence it is of the type of fig. iii. ; the second is an
application of a general principle, and hence is of the
type of fig.
i. The arguments may be expressed syllo-
gistically in Darapti and Barbara respectively :