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

78
of I, the falsity of E :
falsity
IMMEDIATE INFERENCE.
and
from the truth of O, the
of A. But the term Immediate Inference is
usually restricted to certain formal transformations of
which a proposition is capable, and to which Pro
fessor Bain has given the name of
positional
been proposed.
"
equivalent pro-
forms." The name "eductions" has also
There are two fundamental processes of eduction :
conversion, by which we obtain an equivalent pro
position in which S and P have changed places; and
obversion, in which the equivalent has for predicate the
contradictory term
"
"
not-P instead of P. 1
All other
processes of Immediate Inference, in the proper sense of
the term, consist of an alternate performance of these
two elementary operations. Aristotle recognised only
Conversion ; for he did not admit the use of the
definite name
"
not-P as a Subject or Predicate.
9. The term conversion, though sometimes used in
a wider sense, is best restricted to signify the process by
which from a given proposition we infer another having
the subject of the original proposition for its predicate
and the predicate of the original proposition for its
subject. From a proposition of the type SP we infer
an equivalent one of the type PS ; no new term, such
as S (not-S) or P (not-P) is introduced.
The rules for conversion follow at once from the
meaning of the proposition as we have agreed to accept
it. It asserts a relation between two classes. An
affirmative proposition states that two classes are wholly
or partly coincident ( 5, figs, i, 2, 4, 5); a negative
proposition, that they are wholly or partly exclusive of
1 For the future we shall follow a suggestion made by Mr
Keynes, and indicate the logical contradictory of any term P by
the symbol P .
"
in