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

88 IMMEDIATE INFERENCE.
contradictory of the original predicate for its subject,
and the original subject for its predicate. In other
words, we pass from a proposition of the type S P to
another of the type Not-P S, to a proposition giving
us direct information about Not-P.
As before indicated, Contraposition is a compound
operation, involving the two simple operations already
described. To reach the contrapositive the rule is,
first obvert the original proposition, and then convert
the proposition thus obtained. The following table
exhibits the steps and indicates the result in the case
of the four prepositional forms :
Original Proposition.
A. All S is P. No S is P .
E. No S is P. All S is P .
Obverse.
E.
A.
I. Some S is P. Some S is not P .
O. Some S is not P. Some S is P .
Converse of Obverse = Contrapositive.
No P is S. E.
Some P is S. I.
None.
Some P is S. I.
O.
I.
If the previous real examples be taken, "All men
"
are fallible yields as its "
contrapositive No not-fallible
"
"
"
"
beings are men ; No men are perfect yields Some
"
not-perfect beings are men ;
"
Some men are
"
learned
yields no result, because its obverse is an O proposition
and cannot be converted; "Some men are not trust
worthy
"
yields
"
Some not-trustworthy beings are men."
Jevons describes this method of inference, but ap
parently supposes that it is only
A proposition. But he describes
applicable to the
precisely the same