A Truth About A Cube

In this diagram X is non existent in a real cube but is more a drawn effect.

I drew the X cube with using a dot in the centre to align the back side, the Y cube has a square centre to help show you this truth about a cube.

You can never fixate on the back internal corner using the front face - it's impossible.

Therefore, what? I'm stuck here. Does a real cube have a square center, why is X impossible?

With all the current talk on illusion on this board I thought this would be an interesting mathematical debate or discussion.

The draw, climb are two more dimensions. I think. Not like I have much experience with it.