 Here his notion of science is highly significant, because it is this on which his scepticism rests. Science, as becomes clear by the end of That Nothing is Known, has to do with rational demonstrations based on facts or things (rerum) that come to us from the senses (the eyes), albeit imperfectly. What Sanches seems to find most difficult to “swallow” are metaphysical statements and arguments that cannot be proven, except syllogistically by accepting their first principles on faith. Concepts that do not “possess an understanding of natural phenomena [“res intelligas”, QNS: 92]” are “inventions”, says Sanches; “for who could understand non-existent things?” (TNK: 168). The list of such abstract concepts include: “Democritus’s Atom, Plato’s Ideas, Pythagoras’s Numbers, and Aristotle’s Universals, Active Intellect, and Intelligences” (TNK: 168). He addresses himself, he says, only to those who are “‘not bound by an oath of fidelity to any master’s words,’” but rather “assess the facts for themselves, under the guidance of sense-perception and reason” (TNK: 168). Having said this, he ends the introduction to QNS/TNK, just as he did the Poem of the Comet, with the sceptical “Quid?” or What?