The preceding treatment of demonstration relied on certainty. Rhetoric instead allows probable premises. Not every accepted claim is a self-evident principle, and repeated patterns do not by themselves establish strict necessity.
“The whole is greater than its proper part” is treated as a first principle once its concepts are properly understood.
“The weather has usually been mild; tomorrow will probably be mild” is a probabilistic forecast grounded in a repeated pattern.
A proposition’s persuasive appeal and its degree of certainty are different measures.