As soon as they had delivered their message to him, they conducted him to the king, who received him with joy, and, being curious to hear some of his wonderful speeches, commanded him to prophesy. Merlin made answer: "Mysteries of this kind are not to be revealed but when there is the greatest necessity for it. If I should pretend to utter them for ostentation or diversion, the spirit that instructs me would be silent, and would leave me when I should have occasion for it."

Geoffrey of Monmouth's Historia Regum Britanniae