"Columbus discovers Spain" or maybe: whose swimming pool is this actually about? 
Let's imagine the world were blue. Everything, absolutely everything in the wold were blue. Let's say Ives-Klein-Blue. Unexpected possibilities would arise for video-artists experimenting with blue-screens. By means of this thought-experiment , the meaning of the word medium can be clarified: If everything were blue, the colour blue wouldn't be important anymore. The abstract idea of colour would belong to the field of transcendence, because it immanently wouldn't make any sense at all. We wouldn't know at all what blue means. If we were to live in a big cave, tied to poles and could only see the shadows of things, which some random sophists carried in front of torch-lights behind our backs, we wouldn't know anything about the sun; we wouldn’t know anything about these things, and most of all: We wouldn't know what a cave is. The same is true for the colour blue. And it applies to the medium if we look at it as a Behälter* (container) of possibilities. If everything were blue, it wouldn't be useful at all to know what blue is or what colour is, because this knowledge is useless for distinction. There would be no useful distinction for the nature of colour. The knowledge on the medium - the colour, the cave - would only increase its complexity - instead of decreasing it. Educators have made this experience - at least since Plato. But actually it existed since Genesis. The knowledge about good and evil  - i.e. the power of distinction - was bought in exchange for getting kicked out of paradise and meant a massive increase in complexity.