How we cite our quotes: (Book.Chapter.Paragraph.)
Quote #10
Their faces were fanatical, and they babbled of dreams. Ships which moved of their own power, silver tables on which strange Masses had been said, spears which flew through the air, visions of bulls and of thorn trees, demons in old tombs, kings and hermits who had been living for four hundred years—these figured in the rumours which filled the palace. (K.28.2)
The Grail Quest definitely falls into the realm of the supernatural. And by supernatural in this case, we mean God. Literally supernatural: "above nature." It's heavily implied that he's in control of this particular quest, since it's all about teaching the knights to be spiritual instead of worldly.