 Wolff’s ontology is constructed on two foundational principles, namely, the principle of contradiction [PC hereafter] and the principle of sufficient reason. According to Wolff, PC is the fundamental principle of human thought, the very first principle of “all metaphysical first principles”, and the “font [or source] of all certitude” (Ont. §§54–5). It is regarded by him to be a self-evident first principle, its truth made manifest through our inability to think in a manner contrary to it. In the Ontologia, he writes: