That we could not consistently deny our existence without going mad is no evidence for the claim that we do exist—any more than the impossibility of denying consistently that we have free will without going mad is evidence for our actually having it. Why should the truth be believable? For all we know, the true account of what we are might be pathological. That would be a truly absurd situation. It is one thing to accept humbly that our metaphysical nature might remain forever beyond our intellectual grasp. It would be a nasty surprise indeed if we could work out our metaphysical nature all right, but the knowledge of it would inevitably result in madness.

Eric Olson, What Are We? A Study in Personal Ontology, Oxford, 2007, p. 210