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

OPPOSITION OF PROPOSITIONS. 69
Part II. Opposition of Propositions.
5. We have now to examine more closely the mean
ing and use of the four typical forms of the proposition,
A, E, I, O.
We have been treating each of these as affirming or
denying certain attributes of the whole or part
of a
subject. This implies that the subject S is taken in
its full sense, of both extension and intension, that it
means certain objects identified by the possession of
certain qualities ; and the predicate P is read in inten
sion only, it signifies certain other qualities which the
judgment attaches to the subject. "Potassium is lighter
"
than water
: here the subject stands for a real object or
kind of objects, whose qualities we are supposed to know
sufficiently to identify it ; and the judgment predicates
another quality of it, that it will float on water. This
is the natural way of regarding most of our judgments,
and this is apparently the natural meaning of
"
predica
tion." We might therefore express the judgment, as
Aristotle often does, in the form
" P is predicated of S."
We do not usually think of the predicate as a class or
an individual thing, unless we are expressly forming
judgment of classification," as
"
"
a
the whale is a mammal."
This proposition means that the class of animals called
whales is included in the class called mammals.
Now every term has two sides, extension and inten
sion ; hence in every proposition we may read the pre
dicate in extension also, and think of P as a separate
or wider class in which the class S is included. This
is merely a possible way of regarding every proposition ;
but it is the simplest way when we are dealing with
propositions in the manner required by those parts of