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

THE PROBLEMS WHICH WE HAVE RAISED. 333
would be expressed by the traditional Logic, as we have
seen, in a syllogism of the form AAI
Potassium is a metal,
Potassium floats on water ;
Therefore some metals float on water.
Jevons remarks that his own version of the argument
does not omit in the conclusion a part of the premises
i.e., the reference to Potassium ; but we must reply
that it does not give us the conclusion which is of
scientific value. This is true of many other instances
of its
1
application.
The equational theory of judgment and reasoning will
work, and therefore cannot be wholly false ; but it
obscures the true nature of the judgment by making it
a mere identity. We have shown (ch. IV.) that if S and
P are to be strictly identical, the judgment becomes an
assertion of nothing, SP = SP, or a = a. There must be
some difference between the meaning of S and that of P,
and therefore the judgment cannot be expressed in the
form of an identical equation. On the other hand, there
must be some identity between the meanings of S and of
P, for the judgment, as Aristotle said, asserts that they
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are in a sense one (De Anima, III. 6). In what sense
are they one ? They may be one without being identical
in the way in which the two sides of an equation are so.
The two meanings are one in the sense that they are
united in that portion of the real world to which the
judgment refers. Speaking broadly, what the judgment
does is to distinguish S and P in intension, and unite
them in extension. The judgment can combine unity
1
Jevons s
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equational Logic is explained in his Principles of
Science, Introduction ; Studies in Deductive Logic ; and in The
Substitution of Similars (reprinted in Pure Logic and other Minor
Works ].