 Now according to Wolff there are at least four self-evident (axiomatic) principles of human thought: PC, the principle of excluded middle, the principle of certitude (or principle of identity), and PSR (Ont. §§52–55). Of these, only PC is indemonstrable in the sense that the truth of the principle cannot be proved to follow from a formal deductive inference. As we have seen, Wolff believes that we gain assurance of the truth of this principle by attending to the psychological experience of not being able to both affirm and deny our own existence in introspection. Thus only in a weak (and non-Wolffian) sense of “demonstration” can Wolff be said to demonstrate the truth of PC. The remaining principles, however, are demonstrable in the strict sense and each, he believes, can be derived from PC. His demonstration of PSR in §70 of the Ontologia is as follows: