The School of Athens, Raphael, 1509–1511

This is a fresco, not a canvas, painted directly onto a wall of the Stanza della Segnatura, a room in the Vatican's Apostolic Palace that once served as the study of Pope Julius II, who commissioned Raphael for the whole cycle. Nearly every figure in it can be identified as an ancient Greek philosopher, Plato and Aristotle at the centre, yet it decorates the private study of the head of the Catholic Church.

Partway through the work, Raphael reportedly slipped in to see what his rival Michelangelo was painting down the hall on the Sistine Chapel ceiling, a commission running at the same time for the same pope. He was impressed enough to go back and add a brooding, solitary figure sitting on the steps in the foreground, modelled on Michelangelo himself and given the role of the philosopher Heraclitus. A surviving preparatory drawing for the fresco shows the figure isn't there in the earlier version, which is how we know it was a late addition.

Raphael placed himself in the scene as well, on the far right in a dark beret, looking directly out at the viewer rather than at any of the philosophers around him, the one figure in the whole composition addressing the room rather than the debate.

Source: Vatican Museums, Stanza della Segnatura.