Which publication first defined the theatre convention of the fourth wall?

Question

One of the questions on this site asks Do a lot of Shakespeare characters break the fourth wall? However, assuming a "fourth wall" in English Renaissance theatre appears to be anachronistic since

1. spectators surrounded the apron stage on three sides in theatres such as The Globe and
2. the concept wasn't defined until the eighteenth century by Denis Diderot.

Wikipedia writes,

The concept is usually attributed to the philosopher, critic and dramatist Denis Diderot.^[when?]

The comment "^[when?]" was inserted into the article because the actual source in Diderot's writings is not mentioned. Hence my question: Was Diderot really the first to define this concept? If yes, where, when and how?

Answer 1

The reason why the concept is usually attributed to Denis Diderot is the following passage in his Discours sur la poésie dramatique, which contains the following sentences:

Soit donc que vous compoſiez, ſoit que vous jouïez, ne penſez non plus au ſpectateur que s'il n'exiſtoit pas. Imaginez ſur le bord du théâtre un grand mur qui vous ſépare du Parterre. Jouez comme ſi la toile ne ſe levoit pas.

Translation:

Regardless whether you are writing ("composing") or playing, don't think about the spectator, as if he did not exist at all. Imagine that there is a big wall at the edge of the stage that separates you from the pit. Play as if the curtain was never raised.

The "curtain that is never raised" constitutes the "fourth wall", the other three being the stage's back, left and right walls. This assumes a box set as a theatre type, as opposed to e.g. the ancient Greek and Roman theatre, street theatre and theatre in the round.