Si Lewen’s ghost is hanging in my studio — or, more accurately, I should say that one of Si Lewen’s “Ghosts” hangs there. The artist painted about two hundred of these haunted and haunting figures in a series he began in 2008. He gave me my “Ghost” the first time we met, at his rest-home condo near Philadelphia back in the spring of 2013. Si was ninety-four years old then, a dynamo: a charming and elfin man, frail but bubbling with enthusiasm, wry humor, and unorthodox opinions. He spent his long days in the small second bedroom of his apartment —…