It is not an Unknowable behind or beyond everything that we perceive, or can perceive, or conceive in the manner stated.
And the space with which we are concerned is real space, the space in which real things exist and move about, the real things which we can directly know or of which we can definitely know something.

In some sense it must be given in our experience, if the things which are in it, and are known to be in it, are given in our experience.

How must we think of this real space?
Suppose we look at a tree at a distance.

We are conscious of a certain complex of color.

We can distinguish the kind of color; in this case, we call it blue.