When Horace wrote in Ars Poetica, “If you would have me weep, you must first feel grief yourself” (“Si vis me flere dolendum est primum ipsi tibi”), he expressed the ancient world's view that, in order to emotionally affect his audience, an orator needed to feel the emotion himself. This idea was widely subscribed to in the eighteenth century. In the modern era Konstantin Stanislavsky engaged in a sustained investigation of emotion and acting, stressing that the actor needed to experience “real feeling” in order for the audience to experience authentic emotions also. As a theory of emotional transmission, it seems like common sense. Yet, when Denis Diderot witnessed in Baron d'Holbach's salon David Garrick's parlor trick of sticking his head out between two screens, and cycling through a range of passions with his face, the great philosophe wondered whether the actor felt anything at all even though his audience, including Baron Grimm, evidently did. “Can his soul have experienced all these feelings, and played this kind of scale in concert with his face?” Diderot asked, and then answered, “I don't believe it; nor do you.” By deciding in the negative, that Garrick could not have felt anything, Diderot reveals a common fallacy of the audience: the belief that what an audience feels reflects, and is a result of, what an actor feels. The problem for Diderot, which he addressed in the Paradox of Acting (1773), was how an actor such as Garrick managed to evoke emotions in his audience when he apparently felt nothing himself.