When I visited the National Gallery in London last month, I found myself fascinated by minute details in paintings. One type of detail that kept coming up was the bookmark, as in this painting by Lorenzo Lotto:
Holbein the Younger's portrait of Erasmus features not only a stunning depiction of a glass container but also another book with a bookmark in it (in the upper-right corner):
A book alone is apparently not enough to mark a figure as being a humanist (or what we would today call an intellectual?); it takes a bookmark to show that the person is an active intellectual.