The koan is intended to be nourished in those recesses of the mind where no logical analysis can ever reach. . . . All rivers are sure to pour into the ocean; but the koan is an iron wall standing in the way and threatening to overcome one’s every intellectual effort to pass. . . . You feel as if your march of thought had been suddenly cut short. You hesitate, you doubt, you are troubled and agitated, not knowing how to break through the wall which seems altogether impassable. When this climax is reached, your whole personality, your inmost will, your deepest nature, determined to bring the situation to an issue, throws itself with no thought of self or no-self, of this or that, directly and unreservedly against the iron wall of the koan. This throwing your entire being against the koan unexpectedly opens up a hitherto unknown region of the mind. Intellectually, this is the transcending of the limits of logical dualism, but at the same time it is a regeneration, the awakening of an inner sense which enables one to look into the actual working of things. For the first time the meaning of the koan becomes clear, and in the same way that one knows that ice is cold and freezing.

—D.T. Suzuki (An introduction to Zen Buddhism, 1964)