For which of the following forms of categorical proposition is contraposition valid?

1.Some S is P

2.Some S is not P

3.No S is P

4.All S is P

A1, 3 & 4 only

B1, 2, 3 & 4

C1, 2 & 3 only

D2, 3 & 4 only ✓ Correct

Correct answer: (D) 2, 3 & 4 only

Explanation

Contraposition holds for the O, E and A propositions.

For 'All S is P' (A), full contraposition gives 'All non-P is non-S,' valid (Statement 4).

For 'Some S is not P' (O), contraposition gives 'Some non-P is not non-S,' valid (Statement 2).

For 'No S is P' (E), contraposition is valid by limitation, yielding an O conclusion (Statement 3).

The particular affirmative I alone has no valid contrapositive.

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