Best known in his own times as an encyclopedist, the eighteenth-century French writer, philosopher, dramatist, and critic Denis Diderot (1713–84) was to emerge a century later, though his Paradoxe sur le comédien, as a posthumous protagonist in the debate launched in Britain in William Archer's Masks or Faces? (1888). That debate – on the role of feeling and instinct versus craft and technique in acting – has been taken up and sustained by many theorists and practitioners in the succeeding century. In the following article, however, Graham Ley is more concerned with Diderot's wider role as theatrical theorist, suggesting that he offers – as also in his defence of pantomime, his proposal for the ‘serious genre’ which anticipated realism, and his advocacy of scenographic reform – not a unified vision of the nature of theatre but an enduring sense, precisely, of its paradoxical and ironic qualities. Graham Ley has just joined the Department of Drama at the University of Exeter, having previously taught in London and New Zealand. He is currently completing a book on theatrical theory, on which he has previously also published in NTQ, most recently on ‘The Role of Metaphor in Brook's The Empty Space’ (NTQ35, 1993). Among his numerous publications on ancient performance, A Short Introduction to the Ancient Greek Theatre appeared from the University of Chicago Press in 1991.