F. H. Jacobi (1743–1819), a key figure in the philosophical debates at the close of the eighteenth century in Germany, has long been regarded as an irrationalist for allegedly advocating a blind ‘leap of faith’. The central claim of this essay is that this venerable charge is misplaced. Following a reconstruction of what a charge of irrationalism might amount to, two of Jacobi's most important works, the Spinoza Letters (1785) and David Hume (1787), are scrutinized for traces of irrationalism. Far from being an irrationalist, Jacobi is best read as questioning the analytical-geometrical model of rationality popular among his contemporaries, and of proposing a more naturalistic theory of rationality that situates it more firmly in human psychology, the ultimate import of which lies in a reconceptualization of the relation between faith and reason.