We all know that the sky is blue. Grass is green. Blood is red.
But, how do we know that everyone sees these colours more or less the same? How does one know that my green isn't your red and vice versa? In other words: if I would see what someone else sees, would I see the same colours, or could it all be completely different? Note that I'm talking about the perception of colours, not the distinction of colours themselves.
