Painted in 1660 when Titus was 19, Rembrandt’s brush describes not the young man’s physical appearance but his interior life. It is a study not of surfaces or appearances but of thoughts and feelings.

By cloaking Titus in a brown monk’s habit and cowl against a rich brown background, Rembrandt isolates the sitter’s face, making it the whole focus of the picture’s visual interest. But even then not all of the face is shown, for light from an unseen source at the left illuminates the right side, leaving the left partly in shadow.

Titus tilts his head and lowers his eyes, lost in thought. An almost imperceptible smile plays on his lips, and, as in the Mona Lisa, it is this smile that makes the picture so mysterious. Because the sitter is wearing the Franciscan habit, our first thought is that this is not a portrait at all but a representation of St Francis at prayer. But that can’t be entirely true, since this is clearly a portrait. The monk’s habit could also be a studio prop, which Rembrandt gives Titus to wear in joking reference to the order’s vow of poverty, which the boy would have to embrace during the hard financial times the family was going through. Whatever the answer, every brush stroke speaks of the artist’s love for his son, who would die eight years later at the age of 27.