In reply to Steven Bradley.

We see in 2 dimensions, but our brain process the eye signals to give to the object that we are looking at a position in a certain moment; at the saim time we process other information like shape, colours, shadows ecc. with a bottom-up recognition or top-down, the result is a 3-D perception (if I see an object moving in my direction i recognize it and i know that I donothin it will hit me).
The beuty of all this is that we are our brain. We live even if we don't know at all the psicologiy of us.