In case anyone else is wondering: The French phrase can be translated literally as "a canvas requires a wall", or less closely, "its boundaries are important for every picture".
(I am not a native French speaker and just piecing this together with a dictionary.)
Are you sure? I thought it was Renoir or Batut, or Bresson, or perhaps Watteau, who, when asked for his most useful advice to a new artist, famously uttered this short and mysterious phrase. Could have sworn LaBeouf quoted it in an interview after he collaborated with artist Cantor on their magnum opus.
It's so memorable, probably why it stick in my memory: how can you have a canvas without a wall? The wall is the canvas. Yet the wall simultaneously constrains the canvas, thus allowing it to become the canvas, to become worthy of a canvas. This French idiom says so much without saying practically anything.
Even more evidence of how versatile that French phrase is. There's just so many acceptable meanings to it, and every one of them points to the same conclusion: bounds enable art.
This is exactly how all good art is done. There's an old French saying, une toile exige un mur.