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.