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

IMMEDIATE INFERENCE. 93
and the word by which their relation is expressed is
replaced by its correlative.
The traditional Logic recognised only propositions
where the relation between the terms is either one of
the inclusion and exclusion of classes, or the predication
of qualifies of a subject. Hence some recent logicians
(as De Morgan) have developed a scheme called the
"
Logic of Relatives," extending the traditional doctrine
so as to cover the variety of real relations which our
ordinary propositions express.
A few further points require notice before we leave
the general subject of Immediate Inference. First, as
to singular propositions. These have been classed as
universals, and have to be treated accordingly.
"
Brutus
killed Caesar": this is converted into, "Some one who
killed Caesar was
"
Brutus ;
"
St Andrews is an old
university," converse,
" Some one of the old universities
is St
"
Andrews ;
"
Britain is an island," converse,
" Some one of the islands is Britain." If both subject
and predicate are singular terms, the proposition may
be converted simply:
"
St Andrews is the oldest
university in Scotland," converse, "The oldest university
in Scotland is St Andrews." In the case of impersonal
propositions we have, in elementary Logic, simply to
introduce a subject ; thus
"
It rains," is in
logical form
" The atmosphere is letting rain fall," with converse,
"Something letting rain fall is the atmosphere."
In obversion, and therefore also in contraposition, we
assume that both terms, S and P, represent real classes
in the region of fact or thought to which the proposition
refers, and also that not-P is a real class in the same
sense. For instance, take the proposition,
"
All human
beings arc fallible." Its obverse is,
" No human beings