How did Stoic schematic arguments use whole propositions as variables?

Unlike Aristotelian logic, which focused on the internal structure of terms, Stoic logic prioritized propositional logic by treating whole propositions as the fundamental units of inference. To represent these, Stoic logicians—most notably Chrysippus—used ordinal numerals (such as "the first" and "the second") as variables or placeholders for entire propositions.12

This schematic approach allowed the Stoics to formulate principles of logical inference based on how these propositions were combined using truth-functional connectives like "if," "and," or "or." For example, they established five basic inference schemata, known as "indemonstrables," such as: "If the first, then the second; but the first; therefore, the second". By using these placeholders, the Stoics could demonstrate the validity of complex arguments without needing to analyze the internal categorical structure of the individual statements themselves.12