Commentary Purg XVIII 13-15

Why, one might ask, after Virgil's long speech in the last canto, should Dante want Virgil to 'expound' love when he seems just now to have finished (with suitable fanfare) doing precisely that?  What the protagonist means is 'expatiate upon,' 'explain more precisely,' the argument just now presented.  The word dimostrare is here, in fact, a verb based on the Latin noun demonstratio, as used to refer to the development of proofs by means of syllogistic demonstration.  See [Par XXIV 96].