 Part IV begins by rehearsing the defense of metaphysical and scientific knowledge against skeptical philosophy. Following Descartes, it is argued that some items can be so distinctly perceived that they cannot be sincerely doubted. That one must exist in order to think, credited to Augustine rather than Descartes, is cited as an example. “The whole is greater than the part” and similar principles constitute another set of examples which were not much disputed. The same holds for geometrical propositions, but when Cartesians took these as being true of the essence of matter, sceptical resistance was plentiful. Against this, the authors’ only counter is that careful attention reveals the truth as they see it.