There is a Scholastic maxim, contra negantem principia fustibus est argumentandum;but that can scarcely have been the 'direction' which St Thomas had in mind. Philosophia prima here means metaphysics and psychology, culminating in natural theology. There is no better specimen of it than these first three books Contra Gentiles. Accordingly it is defined in book I, chap. I as wisdom.
Of God and His Creatures: 3.25