A medieval Jewish astrologer prophesied to a king that his favourite mistress would soon die. Sure enough, the woman died a short time later.
The king was outraged at the astrologer, certain that his prophecy had brought about the woman's death. He summoned the astrologer and commanded him: "Prophecy, tell me when you will die!"
The astrologer realized that the king was planning to kill him immediately, no matter what answer he gave. "I do not know when I will act die," he answered finally. "I only know that whenever I die, the king will die three days later."