In a second floor apartment in a non-descript building on a street in the Heights section of Jersey City, on a wall likely hidden under coats of paint, is “The Only Poem” by Leonard Cohen. That is its title. “The Only Poem.” I know it’s there — unless, of course, the building has been knocked down or gutted — because I painted it. It was big, maybe three feet wide and five feet tall. I painted it with a small watercolor paintbrush and blue wall paint, starting at about 2 a.m. and finishing about 6 a.m.