Empiricism and Proprioception

There is a long tradition of British empiricist philosophy, going back to Locke, Berkeley and Hume, that claims that the only direct knowledge we have is of our own sense perceptions and that is often skeptical about whether there are objects outside of us that cause these perceptions. These philosophers ignore our proprioceptions: we have direct knowledge about our own bodies that is as certain as our direct knowledge of our sense perceptions.

For example, Bertrand Russell was working in the empiricist tradition when he wrote:

Anything intervening between ourselves and what we see must be invisible: our view in every direction is bounded by the nearest visible object. It might be objected that a dirty pane of glass, for example, is visible although we can see things through it. But in this case we really see a spotted patchwork: the dirtier specks in the glass are visible, while the cleaner parts are invisible and allow us to see what is beyond. Thus the discovery that the intervening medium affects the appearances of things cannot be made by means of the sense of sight alone. [Our Knowledge of the External World, Allen & Unwin, 1922 p.78]

This is an accurate description of how we see things if we have one eye closed, but it is clearly wrong about how we see with both eyes.

If I am near a window with some dirt on it and I am looking out the window with both eyes open, my actual experience is that I can focus my eyes on the specks of dirt on the window and see a blurry, unfocused version of the scene outside, or I can focus on the scene outside and see a blurry unfocused version of the specks. If I want, I can deliberately shift my focus from one to the other, and I have a direct proprioception that I am shifting my focus. If I keep coming closer to the window, it becomes harder to focus on the specks, and I can feel the strain in my eyes. If I keep coming closer, I will eventually come too close to focus on the speck: at first, I will feel the strain increasing as I come closer and keep my eyes focused, and finally I will reach a point where I cannot focus and will see two specks. Then, if I close one eye, I will see only one speck, and if I close the other eye I will see one speck in a different position; I will be able to close one eye and then the other and see the speck move from one position to another or to open both eyes and see the speck in both positions. 

It is not true, as the empiricists claim, that we only have direct knowledge of our sense perceptions and deduce from our perceptions that there is an external world causing them.  In the case of vision, we focus on objects in the external world that are more or less distant in order to have the sense perception. The proprioception that we are focusing on objects outside ourselves is just as basic a bit of knowledge as the visual perceptions.