 Diderot reflected famously and influentially on what he called the “fourth wall of the theatre”, that imaginary barrier that separates an audience from the three dimensions of the stage it faces. This is a barrier that for Diderot acts either as conscious division dividing the actors and the drama from its viewer (theatre as a consciously artificial way of representing and knowing) or as an invisible screen through which the two join together into the joint experience of the theatrical moment (theater as a staged naturalism). While still important in theatre theory today—Richard Sennett interprets Diderot as “the first great theorist of acting as a secular activity” and as the innovator who creates a theory of drama “divorced from ritual”—Diderot’s writings on theatre also offer yet another example of his wider metaphysical and physiological understanding of human beings and their embodied interrelation. They also highlight the role of language throughout his philosophy as a tangible yet permeable and sometimes fragile tether joining humans and their knowing together.