A Class Room Logic / Deductive and Inductive, with Special Application to the Science and Art of Teaching

by George Hastings McNair

The obversion of an A proposition.

1. Principle:

Two negatives are equivalent to one affirmative.

2. Rule:

Negate the predicate and change the A to an E by using the sign no instead of all.

3. Process illustrated.

The Original Proposition (A)	The Obverse (E)
All men are mortal.	No men are immortal.
All maples are trees.	No maples are not-trees.
All teachers should be sympathetic.	No teacher should be un-sympathetic.
All pain is unpleasant.	No pain is pleasant.
All men are imperfect.	No men are perfect.
All birds are feathered animals.	No birds are non-feathered animals.
All men are not-trees.	No men are trees.
All scalene triangles are non-equilateral.	No scalene triangles are equilateral.

The E Proposition.

It is obvious that the process of obverting an E is simply the reverse of obverting an A. Consequently, the same principle obtains; whereas the process may be illustrated by reading the foregoing illustrations reversely.

The rule for obverting E is: Negate the predicate and change the E to an A by changing the sign no to all.