"Oh I agree with that entirely. Dreams within dreams. Happens more often than anyone would dare to think. Dream Crabs are masters of it. They've learned how to weaponise dreams."

The Doctor considers the last comment for a long time in silence. He hadn't really thought of it, because he had always known. But the important question was how had he always known?

"For me, it's no different than the ability to see or hear, or touch. On the one hand, if a dream was trying to establish itself as reality, it would be stupid for it to mess with such inherent senses. On the other, if my brain is somehow aware that this is a dream and trying to tell me it is a dream, that would be a good way to go about it."

Not all that different from what Santa had done when the Dream Crabs were attacking him.