Rembrandt wrote himself in different angles, in all sorts of guises. Sometimes he judged himself with a sober look, noticing the signs of old age on his blue, flabby face, but often liked to appear in various ways: he would dress up as a warrior in armor, then as an impressive king, then as an apostle Paul. Here you can see the features of both styles. The feeble old man is depicted, the time disfigured by his face is almost unrecognizable, much more than on the self-portrait of the same year.
Apparently, he is on the verge of death. But do not forget that this is a portrait of Zeuxis, an ancient Greek artist. He, as is well known, wrote an old woman with a wrinkled face, and her image amused him so much that he laughed unceasingly until he died. The connecting element here, of course, is death, and if this self-portrait is a joke, then it is full of black humor.