Opposition, in the ordinary sense, is the opposition of incompatible propositions, and it was with this only that Aristotle concerned himself. But from an early period in the history of Logic, the word was extended to cover mere differences in Quantity and Quality among the four forms A E I O, which differences have been named and exhibited symmetrically in a diagram known as: The Square of Opposition.
The four forms being placed at the four corners of the Square, and the sides and diagonals representing relations between them thus separated, a very pretty and symmetrical doctrine is the result.
Contradictories, A and O, E and I, differ both in Quantity and in Quality.
Contraries, A and E, differ in Quality but not in Quantity, and are both Universal.
Sub-contraries, I and O, differ in Quality but not in Quantity, and are both Particular.
Subalterns, A and I, E and O, differ in Quantity but not in Quality.
Again, in respect of concurrent truth and falsehood there is a certain symmetry.