De Interpretatione Ch. 6 - "Affirmative and Negative Propositions Have Opposites" - Aristotle

Affirmative and Negative Propositions Have Opposites

We mean by affirmation a statement affirming one thing of another; we mean by negation a statement denying one thing of another. 

Aristotle introduces the concept of "quality" in regard to statements. There is the affirmative quality and the negative quality. The affirmative unites the subject and predicate and the negative separates the subject and predicate. 

As men can affirm and deny both the presence of that which is present and the presence of that which is absent and this they can do with a reference to times that lie outside the present, whatever a man may affirm, it is possible as well to deny, and whatever a man may deny, it is possible as well to affirm. Thus, it follows, each affirmative statement will have its own opposite negative, just as each negative statement will have its affirmative opposite. 

Regardless if it's past, present, or future, one can still offer a qualitatively opposite statement to the one given. 

We Must Compare Equal Sentences

Every such pair of propositions we, therefore, shall call contradictories, always assuming the predicates and subjects are really the same and the terms used without ambiguity. 

Statements are opposites only when we make sure that subject and predicate are the same, and the terms are defined clearly. 

These and some other provisos are needed in view of the puzzles propounded by importunate sophists. 

These distinctions are needed because people use fallacious arguments which get confusing.