See Part II, VIA QUAESTIONIS, b. Deductive Demonstration (Proof) (Syllogism of the first figure) .
See also NOTE 157 in Part IV (press BACK button of browser to return).
The Syllogism is a certain scheme of arguing : a maior (1st premise), a minor (2nd premise), and a conclusion. A premise is a proposition the truth of which is conceded by the participants to the argument.
A petitio principii is a circular proof (that is, the conclusion was already contained in one or both premises, and such a proof [favorite among those who want to persuade innocent people of the truth of some theological doctrine] is consequently invalid.