Rambam (Introduction to Mishna) One should not think, "Since a prophet proves he is genuine by the fact that his predictions come true, then all fortunetellers, astrologers and mind readers can claim to be prophets, because we see with our own eyes that they predict the future every day!"…I want to put it this way: Fortunetellers, astrologers and all others in this category do indeed predict future events, but their predictions are only partially true, while the remainder inevitably turns out to be false. We see this happen all the time, and even the fortunetellers themselves do not deny this. The only way that one fortuneteller is better than the next is that he told fewer lies than his colleague did. But that all of a fortuneteller's predictions should come true is simply impossible. These crystal gazer do not even pretend or boast that their predictions will come true in all details. One fortuneteller might say, "This year there will be a drought; there will be no rainfall at all this year," when in fact a small amount of rain will fall. Or another might say, "Tomorrow it will rain", while it actually will rain on the day after. And even such a near miss will happen only to an eminent expert in the art of the occult, one of the authorities who are quoted in books. Yeshayah spoke about these diviners when he challenged Babylonia, "You are helpless despite all your art. Let them stand up and help you no, the astrologers, the stargazers, who predict parts of the months' events, whatever will come upon you. Our Rabbis commented that Yeshaya stated, "who predict parts of the events", and the he did not state "who predict those events in their entirety". The testimonies and predictions of the prophets however, are altogether different. All their prophesies come true word for word and letter for letter, and not even the slightest detail of anything they say in the name of HaShem remains unfulfilled… This is what Yirmiyah had in mind when he said that the visions of people who claim to have prophetic power must be proven to be correct in every detail. He would discredit the false prophets and demolish their claims to prophecy, saying, "Let the prophet who has a dream tell the dream; and let him who has received My word report My word faithfully! How can straw be compared to grain?" says HaShem (Yirmyah 23:28). The Sages explain this to mean that prophecy is crystal clear without any elements of falsehood blended in, just as grain kernels are separate from straw. The dream and similar things that are based on interpretations are mixed with untruths just as straw that contains kernels of wheat. "Just as it is impossible to have grain without straw, so too it is impossible [for an ordinary person] to have a dream without senseless matters"
