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Seeing as he is a Navi Sheker, how come that the sign that he gave came true?
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Seforno and Rashbam: Because he employed witchcraft or something similar (a Ru?ach Tum?ah - Terafim [household idols] or the sorcery of Ov or Yid?oni, and Hashem gave them the power to know the future in order to test Yisrael, to test Yisrael ? Rashbam). 1
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Rashbam: As to whether hey will fulfill the Mitzvah of ?Lo Yih?yeh b?cha Me?onein, u?Menachesh u?Mechashef ? Tamim Tih?yeh im Hashem Elokecha? ? Parshas Shoftim, 18:10-13.