Whether a categorical proposition is affirmative or negative concerns the form of its judgment; whether it is true or false concerns the correctness of its content. These are independent questions.
“A human is an animal” — affirmative and true.
“A human is a stone” — affirmative but false.
“A human is not a horse” — negative and true.
“A human is not an animal” — negative but false.
Thus affirmative propositions need not be true and negative propositions need not be false.
Opening interactive controls...
Chapter Two: Propositions, Categorical and Conditional Forms, and Their Parts
Whether a categorical proposition is affirmative or negative concerns the form of its judgment; whether it is true or false concerns the correctness of its content. These are independent questions.
“A human is an animal” — affirmative and true.
“A human is a stone” — affirmative but false.
“A human is not a horse” — negative and true.
“A human is not an animal” — negative but false.
Thus affirmative propositions need not be true and negative propositions need not be false.