How do affirmative propositions differ from negative propositions in Aristotelian syllogistic?

In Aristotelian syllogistic, the difference between affirmative and negative propositions is defined as a difference of quality.12

Categorical propositions are analyzed as consisting of a subject, a predicate, a copula, and sometimes a negation. Affirmative propositions assert that a predicate applies to a subject, while negative propositions deny that application. These are categorized into four basic forms, historically designated by the vowels in the Latin terms affirmo (I affirm) and nego (I deny):342

• A (Universal Affirmative): "Every S is P"45
• E (Universal Negative): "No S is P"45
• I (Particular Affirmative): "Some S is P"45
• O (Particular Negative): "Some S is not P"45

The quality of a proposition—whether it is affirmative or negative—is distinct from its quantity, which refers to whether the proposition is universal or particular. These qualities determine how propositions relate to one another within the square of opposition, such as being contraries, contradictories, or subcontraries, and dictate the rules for valid logical inference.1235