The
fourth wall is the imaginary "wall" at the front of the stage in a traditional three-walled
box set in a
proscenium theatre, through which the audience sees the action in the world of the
play. The concept is usually attributed to the philosopher, critic and dramatist
Denis Diderot. The term itself was used by
Molière. The fourth wall illusion is often associated with
naturalist theatre of the mid
19th-century, and especially with the innovations of the French director
André Antoine.