We are looking at a picture in which the painter is in turn looking at us. A mere confrontation, eyes catching one another’s glance, direct looks superimposing themselves upon one another as they cross. And yet, this slender line of reciprocal visibility embraces a whole complex network of uncertainties, exchanges, and feints. The painter is turning his eyes towards us only in so far as we happen to occupy the same position as his subject.
No gaze is stable, or rather, in the neutral furrow of the gaze piercing at right angle through the canvas, subject and object, the spectator and the model, reverse their role to infinity. And the great canvas with its back to us on the extreme left of the picture exercises its second function: stubbornly invisible, it prevents the relation of these gazes from ever being discoverable or definitely established. The opaque fixity that it establishes on one side renders forever unstable the play of metamorphoses established in the center between spectator and model.
As soon as they place the spectator in the field of their gaze, the painter’s eyes seize hold of him, force him to enter the picture, assign him a place at once privileged and inescapable, levy their luminous and visible tribute from him, and project it upon the inaccessible surface of the canvas within the picture. He sees his invisibility made visible to the painter and transposed into an image forever invisible to himself.
Michel Foucault, The Order of Things (1966).