In fact, humans are known to have the largest and most visible sclera - the whites of the eyes - of any species. This fact intrigues scientists, because it would seem actually to be a considerable obstacle: imagine, for example, the classic war movie scene where the soldier dresses in camouflage and paints his face with green and brown color - but can do nothing about his noticeably white sclera, beaming bright against the jungle. There must be some reason humans developed it, despite its obvious costs. In fact, the advantage of visible sclera - so goes the cooperative eye hypothesis - is precisely that it enables humans to see clearly, and from a distance, which direction other humans are looking. Michael Tomasello showed in a 2007 study that chimpanzees, gorillas, and bonobos - our nearest cousins - follow the direction of each other's heads, whereas human infants follow the direction of each other's eyes. So the value of looking someone in the eye may in fact be something uniquely human.

In one of the most famous passages of Being and Nothingness, The Look, Jean-Paul Sartre describes the peculiar vulnerability that develops when someone goes from seeing (being a self with a perspective on the world) to being seen (having to confront the perspective of another on one's self). He illustrates it with the example of someone looking through a keyhole who suddenly finds himself caught by someone watching him. The look of the other is always unnerving, Sartre argues, not only because we momentarily recognize ourselves in it through our imagination of their judgment of us but also because we don't. We can always step back, challenge our perception of others' perceptions of ourselves, or explain them away - but we don't know what these perceptions really are. Others have the distinctive power of making us feel judged in ways we cannot fully control. Social life is all about the fear that accompanies our awareness that we can never access what the other sees. We can only guess.