Sade’s writing is infamous; it is widely referred to as if its meaning is obvious and may serve a broader point. This is done most often in twentieth-century critiques of the Enlightenment: in the “Continental” tradition by Adorno and Horkheimer, Lacan, Bataille, de Beauvoir and Foucault; it is also done in Anglophone Intellectual History including by Lester Crocker who identifies Sade with the Enlightenment in order to show it as a slippery slope of nihilism which leads inevitably to Sade, Robespierre, Nietzsche and Hitler. Few people have examined Sade directly and attempted to understand his philosophy in its own terms. This paper proceeds as contextualist history of philosophy examining Sade’s oeuvre as it engages with the philosophes and with Rousseau. Taking epistemology as primary I will argue that the Enlightenment’s medical anthropology is the key to understanding its “knowing body”. This brings together the period’s Lockean-derived Sensationism with its theories of moral sense. It is on this ground that Sade enters into a sophisticated and precise philosophical engagement with his philosophical context. I argue that understanding of this engagement is the key to understanding Sade in his own terms.