The Square of Opposition

What is contrariety?

"A" and "E" propositions. Contraries cannot both be true, but they can both be false.

Subcontraries are the

"I" and "O" propositions Subcontraries can both be true, but cannot both be false.

What are the 4 different kinds of opposition?

Contradiction. Contrariety. Sub-contrariety. And sub-alternation/superalternation

Subalternation is the relation between the

Opposed "A" and "I" propositions and between the Opposed "E" and "O" propositions. If universal proposition is true, then the particular proposition is true. If the particular proposition is not true, then the universal cannot be true.

To refute an "A" or "E" proposition (I.e. Universal) you do not need to

Prove its contrary, only its contradictory.

The 2 sets of contradictories are

The "A" proposition and the "O" proposition. All s is p, and some s is not p.

Opposition

The relation between any two propositions that have the same subject and predicate, but differ in quality or quantity or both.

Propositions that are in opposition have

The same subject and predicate.