Each day when you wake up, the world is, for the most part, unchanged from the day before.
The sun rises again in the east. Your underwear falls if you drop it. The water in the sink spirals down the drain like always. Just as important, your mattress won't turn into a sports car and you can't jump into the air and fly like Superman.
Reality, in other words, seems pretty stubborn, pretty fixed — and pretty much independent of whatever is going on in your head.
But is it? Is it really all those things?