'Affirmative or negative' is a classification of propositions on the basis of their

Whether a proposition affirms or denies its predicate is what logicians call its quality.

An affirmative proposition joins the predicate to the subject; a negative one separates them.

This joining or separating force is carried by the copula, "is" or "is not".

Quantity is a different basis, concerning how much of the subject is taken.

The affirmative-negative division therefore turns on quality alone.