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

(4)
IMMEDIATE INFERENCE. 8/
"A man s a man."
(a) a human being ;
(b) a being with the capacities and rights of manhood ;
(c) affirmed of every instance of the subject.
Hence SaP,
"
All human beings are beings
capacities and rights of manhood "
; obverse SeP ,
with the
"
no
human beings are other than beings having the capacities
and rights of manhood."
(5)
"
Britain is an island."
This is a singular proposition, and therefore SaP. The
"
obverse is SeP ;
"
Romulus and Remus were twins."
(6)
Britain is not other than an island."
(a) Romulus and Remus (a singular collective term) ;
(b} twins ;
(c) affirmed of the whole subject.
Hence SaP, "Romulus and Remus are twins," with
obverse SeP ,
"
Romulus and Remus are not other than
twins. (Cp. 4, Ex. II.)
EXERCISE V.
Give the obverse of each of the propositions referred to
in Ex. III.
Before passing from this subject we must add a note
on the so-called "geometrical
obverse." The geomet
rical obverse of "All S is
"
P
is "No not-S is P," which
is not logically inferrible from the former, and requires
independent proof. It is true whenever the classes of
things signified by S and P are coextensive, as in 5,
fig. 2.
ii. Other processes, of genuine Immediate Infer
ence, consist in combining Conversion and Obversion.
We shall examine two such processes, Contraposition
and Inversion.
Contraposition is the process by which from a given
proposition we infer another proposition having the