I think of Art, at its most significant, as a DEW line, a Distant Early Warning system, that can always be relied on to tell the old culture what is beginning to happen to it.

Marshall McLuhan (1911–1980) Canadian philosopher, communication theorist, educator

Quoted in Richard Schickel, “Marshall McLuhan: Canada’s Intellectual Comet,” Harper’s Magazine (1965-11)
    (Source)

Based on "conversations" Schickel had with McLuhan.

Often cited to McLuhan's breakout work Understanding Media (1964) (e.g., here and here), but not found there.