According to the Aristotelian philosophy of science, the principles of demonstation do not themselves need to be proved. Why not? What is peculiar to them? To show that, Scholasticism makes use of a phrase derived from Boethius: the principles are "per se nota" ("selbstverständlich" in the German translation adopted in this work). Thomas Aquinas in particular addresses in a few telling passages in his works the conditions that make a proposition "per se nota". This work tries to clarify the conditions stated by Aquinas. On the one hand it illustrates the context in which these conditions are introduced, namely the question of the self-evidence of the existence of God; on the other hand, it deals with the historical background and the philosophical prerequisites of Aquinas' position. All this not only provides a historically based interpretation of Aquinas' thought, but also offers a rich supply of material to reconstruct an important chapter in the history of ideas.