Popper's negative model of reason, in which reason only discards falsehoods, is highly relevant to the current debates on rationalism. Popper rejects foundationism, identifying reason with the duty and the privilege of grounding (proving or justifying) knowledge claims. His arguments against foundationism, as well as his proposed solution, can be tracked back to the dispute with Hume who, as Popper observes, drew the wrong conclusion from the fact that reason cannot justify knowledge, and proceeded to undermine the authority of reason and its normative function in epistemology. Today, Popper's criticism of Hume can be applied to the attempts of postmodernism to devalue reason. His conception of ratio negativa has the potential to uphold the value of reason while, at the same time, acknowledging that postmodernists target an important problem: the Western culture has been, for centuries, under the rule of the foundationist, authoritarian rationalism that should be abandoned.