Wilson, Douglas, and Nathan D. Wilson. The Rhetoric Companion: A Student's Guide to Power in Persuasion. Moscow, Idaho: Canon, 2011.
Lesson 12, “The Structure of Argument” pp. 63-66.
In this lesson Wilson introduces the reader to the “square of opposition, which will bring us, in due time, to the categorical syllogism” (Wilson 2011, 63). The square of opposition helps us analyze whether specific statements are true or false. On p. 64 Wilson describes this square.
At the top are universal statements
A always true
E always false
And at the bottom are particular statements
I sometimes true
O sometimes false
The universals are on top, particulars on bottom. The positives are left, negatives right.
On p. 65 Wilson describes the relationships.
A and E cannot both be true but can both be false.
A and O cannot both be true and cannot both be false.
E and I cannot both be true and cannot both be false.
A and I - Because A is true I must be true.
E and O If E is true O must be true.
I and A If I is false then A must be false.
O and E If O is false E must be false.