In the ancient meditation instructions, it is said that at the beginning, thoughts will arrive one on top of another, uninterrupted, like a steep mountain waterfall. Gradually, as you perfect meditation, thoughts become like the water in a deep, narrow gorge, then a great river slowly winding its way down to the sea, and finally the mind becomes like a still and placid ocean, ruffled by only the occasional ripple or wave.
—Tibetan Book of Living and Dying, chapter 5