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In his Mishneh Torah (Hil. Yesodei HaTorah 10:4), Rambam speaks of the guidelines for establishing whether one who claims to have received prophecy is a genuine prophet. Rambam writes that if the prophecy the person delivers foretells some future act of compassion, mercy or beneficence on the part of God, and this event does not materialize, then the supposed prophet is surely a charlatan, because God would never renege on a promise of good tidings. If, however, the prophecy promises bad tidings—famine, destruction, or the like—then the failure of the prophecy to materialize cannot be taken as evidence that the prophet is a liar. This is so even if the prophecy of bad tidings was given unconditionally, because, Rambam argues, there is no such thing as an unconditional promise of bad tidings; the possibility of reversal always exists, even if the prophet doesn’t explicitly state this. In essence, Rambam seems to be suggesting that prophecies of bad tidings ought to be seen as cautionary warnings of a worst-case scenario that will take place unless steps are taken to avert it. Thus, when Jonah prophesied that in forty more days Nineveh would be destroyed, his prophecy must be seen as a warning to Nineveh that their destruction looms, unless it is averted by the force of human free will; in the event, it is averted, after the people of Nineveh choose to change their ways.
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