In his Introduction to the Elementatio theologica, Joane Petrizi reflects on the sense (the intentio) of this book, and sees it in the proof of ‘the much discussed One’ (Petrizi, Commentaries, p. 149). Then he briefly treats the peculiarity of the One, which in his opinion is not identical with any being. The pure and genuine One must be examined and proved in accordance with the rules of syllogisms (Petrizi, Opera II, p. 3). According to Petrizi’s exegesis, this One is the crucial concept in the overall process of grounding knowledge. If this principle is not provable, there are no incontrovertible propositions and the goal of gnoseology will not be reached. The One is the principle that makes knowledge possible.