How many basic logical forms are there in the traditional categorical system?

• A. Three
• B. Five
• C. Four (A, E, I, O)
• D. Six

Answer: (C)

Four basic forms exist in the traditional categorical system. They are universal affirmative (All S are P), universal negative (No S are P), particular affirmative (Some S are P), and particular negative (Some S are not P). These four cover every way a subject term can be attributed with a predicate, depending on whether the statement is about all members or some members, and whether it affirms or denies the relationship. The square of opposition helps illustrate how these forms relate to one another—contraries, subcontraries, and contradictions—showing they form a complete, distinct set. So, there are four basic logical forms.