Technically, of couse, we only perceive what is not there: we experience the world indirectly, through neurological experiences that inevitably lag, ever so slightly, behind the events which trigger them. We trick ourselves into believing that we can perceive what is really there if only we pay close enough attention. This is hard to do with sound (whatever we have heard is gone as soon as we hear it) and easier to do with pictures. We pretend that we can see what is there.
We also pretend, of course, to see what is not there.This is partly to notice that we observe figures and ground, and the separation that tells us which is which is what is not there and yet the focus of our attention. Think about that slowly. The figure is what we are looking at and the ground is the setting in which that figure appears. No figure ever appears without a ground, and we never look at a ground without making some part of it the figure. Try it: look out your window and notice what you notice. You will see the window frame, or the dog being walked, or the window across the street. And you will be aware that each of these things occurs in the context of other things - the ground, the visual information which remains within reach and outside of awareness.
How do you know where the figure ends and the ground starts?. . . finding the edge of the figure is as simple and as finicky as finding the edge of the roll of tape or the place where a story begins. What determines the edge is a relationship between the observer, the figure and the ground. Relationship is always observable and always invisible.