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

332 THE PROBLEMS WHICH WE HAVE RAISED.
Jevons s
"Equational Logic" resembles in principle
the Symbolic Logic of Boole and Venn. He holds
that the real meaning of every proposition is to assert
that subject and predicate are identical. With some
propositions this is evident in expression, in A pro
positions which can be converted simply e.g., "All
equilateral triangles are equiangular." When this
"
simple
"
identity is not actually expressed, Jevons
holds that the proposition must be expressed so as to
show it. Instead of saying "All S is P," we must say
"
All S is SP," for S can be identified only with the Spart
of P and then we ; may write the proposition S =
SP. Reasoning consists in putting propositions together
and drawing conclusions from them (ch. I.
7)
: this
process, in Jevons s system, becomes the "Substitution
"
of Similars
i.e., the substitution in one proposition of
the value of a term as given in another. Take a simple
example. We want the conclusion from the proposi
tions "Potassium is a metal," and "Potassium floats
on water." We write them thus
1 i ) Potassium = Potassium metal ;
(2) Potassium = Potassium which floats on water.
Substituting for
on the right-hand side of
"
"
potassium
(i) its value as given in (2), we get
(3) Potassium = Potassium metal which floats on
water ;
or, in ordinary language,
"
Potassium is a metal which
floats on water." When the terms are numerous and
complicated, real simplification may be obtained by
these means.
It may be said that in the example given, the con
clusion which is both natural and scientific would be
that metallic attributes do not exclude the degree of
lightness necessary for floating
on water. And this