Ernst von Glasersfeld's contribution is elegant and stable. It forms a solid foundation for understanding our own experience and appreciating other's. My wholehearted concurrence needs to be acknowledged up front. Essential disagreement will have to be left to others. My comments will focus on the logical foundations and consequences of radical constructivism, its ties to science, and its uses and implications.
Experiential space surrounds the viewpoint like a bubble.
Whenever 'I' am here, the bubble is also. The content within the space changes but the space itself doesn't. On the Cartesian view, of course, this single experiential space is severed into a viewer and a world viewed. From within that model, because it acts as a filter, the physical world is primary. The brain responds to sensory input from that world and then represents it to one as experience.