Mona Lisa and Trompe l'œil

Trompe l'œil means a trick of the eye - art that deceives the eye - and it became popular from the 15th century. Leonardo is not really associated with it in any formal sense, aside from The Last Supper, but how else can we debate the Mona Lisa smile? People are fascinated by the idea that there might be hidden meaning here... Who is she and what is she smiling about? Is she a Sphinx?

The trick of the eye is that Leonardo's painting looks straight back at the viewer (as the St. John the Baptist painting does) and there is a trace of irony in that glance, and this unsettles those who think that art should not look back. They convince themselves there is something sinister there.

Not sinister but the infinite...

There is a concept that is the corrupter and destroyer of all others. I speak not of Evil, whose limited empire is that of ethics; I speak of the infinite.
- Jorge-Luis Borges.

Leonardo was fascinated by it, by what is unpaintable - astrapen, bronten, ceraunobolian.

Above: a conventional trompe l'oeil - the Museo della Musica in Bologna, Italy. Credit: Jdk at it.wikipedia. Below, a subtler one: Holbein's The Ambassadors (1533):

The trick of the eye is the strange elliptical object in the foreground, which turns out to be an anamorphic (stretched) image of a skull - a puzzle set for the viewer. The skull was a traditional symbol in painting for human mortality in the 16th century - memento mori or vanitas - and you can see another example here.