I see a wall. The wall is curved. The wall is rough. And the wall is red.
A cloud passes over and suddenly the wall appears to be flat. It appears to be smooth. And it appears to be violet.
Now, I can walk over to this wall and say to myself... Well... I can reconfirm that it's curved by walking along it. I can run my hands over it it and reconfirm to myself that, in fact, it was rough. But what do I do about violet? Is violet a mystification of some kind? Is it a lie? And if the cloud should go away and it becomes red... do I think to myself: 'Was it red all along?'