 PC is the “font of all certitude” insofar as, if it were called into question, the most evident and secure judgments of human knowledge, such as knowledge of the self (as a thinking thing), could likewise be called into question. We recognize the fact of our own existence by recognizing the psychological impossibility of denying it. But if it were possible both to affirm and also deny our own existence (simultaneously), then the experience of certitude that accompanies this cognition would thereby be undermined.