Most of the standard discussions of vision in Berkeley’s day were couched in terms of geometric diagrams showing how light rays converged and diverged when they passed through lenses or were reflected from surfaces of varying curvature. It was claimed that distance was estimated on the basis of the angle at which light entered the eye. His criticisms of such views give the first insight into the special character of Berkeley’s concern. He says that the perception of distance cannot be explained by lines and angles because we never perceive any such things and those who know nothing of optics perceive distance without ever thinking of such lines and angles. Berkeley wants to know what we perceive immediately which allows us to say that something is near or far away. Nor will he allow us to say we perceive the distance between us and an object. We estimate it on the basis of our immediate perceptions. There are three we typically use: first, the sensation we get when we cross our eyes to see something very close; second, the confused appearance of an object as it gets close to the eye, and third, the muscular strain involved in preventing, temporarily, the confused appearance of an object close to the eye. In addition to these we use our knowledge of the size, number, kind, and so on, of the objects in question. There is no necessary connection between these perceptions and the distance of objects. We have found the connection in experience and this gives rise to a habitual or customary connection between these two kinds of ideas. Visual perceptions are signs of distance and are related to it in the way a blush is to shame, or a word to the idea it stands for.
We recall that there was a national hysteria, only a few years ago, over the fact that Sarah Palin’s website published a graphic in which Democratic congressional districts chosen for Republican challenges were marked with crosshairs. President Barack Obama himself felt the need to weigh in on the issue, calling for — this seems quaint — “civility.” Sarah Palin has never done violence to anything other than the occasional moose, but faced with actual political violence, the people who still insist on calling themselves “liberals” are strangely quiet.