Comments and expansion
Today again, Sam Harris strictly speaking asks for the impossible. Or does he not?
Focus on the intervening space. Notice how you can see the space itself. It's almost as though the air is shimmering.
The context: in this meditation, Sam Harris uses the visual sense, and in particular the wide-open gaze, to first construct a duality between seer and seen ("place yourself at the edge of the visual field") and then to deconstruct it using his typical search operation resulting in unfindability ("see if there is a seer, or whether there is only seeing").
At first glance, one may think that focusing on space is impossible. The problem: there is nothing to "focus" on. In particular not, when we follow Sam´s other instruction to "leave the gaze wide open".
It´s either focused attention or peripheral holistic awareness.
There is nothing in space (as space) that one could "focus" on. We cannot focus on air molecules; even if we could, these would not be the space within which the molecules float.
Space cannot be focused on as such if we define focusing as "focused attention on an object". Space is not an object, it is the container for all objects, just like consciousness itself.
So, maybe Sam Harris uses this construct ("Focus on space") as a further means to do the impossible and thereby deconstruct the seer.
On the other hand, maybe he just uses the term "focus" lightly, rather than in the neurologically sound sense.
Maybe he says this from the point of view of a highly trained meditator. Such a meditator can vary the scope of attention (of focus) intentionally such that the scope eventually encompasses everything, including itself.
You can try to vary the scope of your focus. Just look at any object in the room with a laser-sharp focus. For example, if you look at a picture, look at an individual object in that picture.
And then slowly expand the scope gradually, until the entire picture is in your focus.
And then you can expand the scope further and further out.
In Culadasa´s terms, one can expand the scope of focused attention such that it eventually covers all objects in awareness, and the distinction between focused attention and peripheral awareness becomes moot.