Ferdinand Tönnies, the founder of sociology, has been characterised as contributing to the “destruction of reason,” although he viewed himself as a champion of Enlightenment with a social vocation. Here, we shall consider Tönnies’s discussion of the epistemological bases of what he called “rationalism”: his theory of the state, based on the rationalism of Hobbes, and of society, based on the philosophers of the Scottish Enlightenment, Smith and Hume; his implicit development of rationalist ethics and the positions he took on (...) Spinoza and Kant; and the relationship between his philosophy of history and that of his philosophical forebear, Adam Ferguson, who wrote in the age of bourgeois emancipation that marked the high Enlightenment. We shall conclude with reflections on the grounds on which Tönnies has been read as an opponent to rationalism and even as a foe of Enlightenment. (shrink)