Square of opposition

The Square of opposition is a term from the study of Aristotelian logic, in which the logical relationship between various types of sentences is spelled out:

These rules apply:

1. At least one of the universal statements must be false.
2. Contradictory statements have opposite truth values.
3. Universal statements entail their subalterns.
4. At least one of the particular statements must be true.

Only the first two rules are explicitly stated by Aristotle (in his work, De Interpretatione) but the other two can be inferred.