While poking around looking for stuff related to the Zeuxis and Helen post (see below), I came across a Rembrandt self-portrait that I had seen many, many, many times before, but didn't see the ClassCon in it. The portrait, which comes from the end of Rembrandt's career, has a sort of 'ghostly figure' in the upper left. According to the Web Gallery of Art (whence the image comes), that ghostly image is probably a sculpture of Heracleitus which Rembrandt is known to have had in his studio.