He implies that when there is no timeless truth to be established, or when claims to know it are essentially contested, then there is no proper argument but only persuasion. This overstates the case against non-Platonisms radically, but Mason seems to mean it. This is an oversimplification of Nietzsche and perspectivism. Even perspectivists can argue deductively in the sense of restating their premises tautologously. Fortunately, Mason has two stronger arguments for the inseparability of philosophy and rhetoric.
Argument works only where there are shared premises.
Hence philosophers turn to rhetorical devices, particularly metaphor, again and again to establish the conditions in which their strict arguments can move minds. This strong thesis, implying that argument has no force of its own, would quickly establish the inseparability of philosophy and rhetoric, but Mason does not elaborate upon it. Since writing is addressed to audiences, philosophers writing for fractious audiences will leaven their arguments with rhetorical tropes. Philosophical audiences have been disputatious and disagreeable from the beginning. Historically this fact of life is contingent, but for us it is universal.