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

IMPORT OF PROPOSITIONS AND JUDGMENTS. IOI
extension only ; in reading them in extension we must
have a reference to intension.
We adopt the class view in the third and fifth chapters of
this book, because that theory has sufficient truth to work
for the purpose to which it is applied. The whole doctrine
of Immediate Inference and Syllogism may be stated in
terms of the first, of the second, and of the third views of
the proposition ; but the second view simplifies those
doctrines so much that there need be no hesitation in
keeping
to it.
If, however, it is insisted that the proposition shall be
rigidly interpreted in extension only, the result is to turn
it into a form of words which states nothing. This
result is reached in two steps, (a) It is not sufficient
to say that
"
All S is some unless we P," specify that
the S-part of P alone is meant,
for on the exclusive
class view, as the diagrams show, the copula
"
"
is
means "is identical with," "coincides with." In saying
that "All men are some mortals," we should specify
what "some" is meant; "some" stands for the human
part of
"
mortals." Hence, looking simply at the side of
extension, we get the equational view of the proposition
upheld by Jevons in his larger logical works ; we get
"
all men are men-mortals," not merely
"
some mortals."
Jevons distinguishes the class of A propositions, which
are simply convertible, as
"
simple identities
"
e.g.,
"The Pole Star the star which moves most slowly";
all others he reduces to the form S = SP in order to
produce an equation. () In such a proposition, the
one side differs from the other only by the addition of
P. But if this constitutes a real difference, we must
add P to the first side also, and say, SP = SP, "Mortal
men are mortal men," which is a proposition telling us
nothing. To make the terms S and P signify their