Immediate Inference:
Obversion

When we group things together into a class, we are distinguishing them from everything else. For every class C, there is a complementary class composed of everything else, everything not included in C. This is called the complement of C, and it is usually labeled "non-C." Thus the complement of "abrasive things" is "nonabrasive things."

We arrive at the obverse of a proposition by making two changes:

1. Replace the predicate term with its complement

2. Change the quality of the proposition (affirmative to negative or negative to affirmative)

Example:
STANDARD PROPOSITION
OBVERT
All S is P No S is non-P
All sandpaper is abrasive.
No sandpaper is nonabrasive.
No S is P All S are non-P
No machine is conscious.
All machines are nonconscious.
Some S are P Some S are not non-P
Some people are kind.
Some people are not unkind.
Some S are not P Some S are non-P
Some chemicals are not toxic.
Some chemicals are nontoxic.


Conversion | Obversion | Contraposition

Immediate Inference

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