Hume and the Limits of Reason

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So begins Treatise I iv 1, Of Scepticism with Regard to Reason. The rules of reason may be perfect, but we are not. From this truism Hume cultivates a notoriously obscure argument to the effect that we should never trust our reasoning. In characteristic fashion, Hume considers the argument sound but unpersuasive. He believes that no one who reflects on it will lose faith in their reason and sets out to explain why. In this essay, I am concerned to make two main points. First, the argument is more successful than often thought; and second, reflecting on Hume's explanation for its lack of force illuminates the depth of his naturalism and contains an important insight into the limits of human reason.2 Hume actually presents two distinct arguments in Of Scepticism with Regard to Reason.3 The arguments are distinct insofar as they are aimed at different conclusions; they are related in that they share a premise concerning our natural fallibility as reasoning agents. The first argument (T 181) is meant to show that any belief formed in the demonstrative sciences—any a priori belief about (say) mathematics—cannot be held with certainty. At best, such beliefs can be taken as probably true. This argument can be sketched as follows.4 In performing any set of calculations, no matter how simple, we are