Reading , Spinoza wrote that when Abraham heard God's promise, he demanded a sign, not because he did not believe in God, but because he wished to be sure that it was God Whom he heard. Spinoza concluded that prophecy itself did not afford certainty, and the prophets were assured of God's revelation not by the fact of revelation itself, but by some sign. Spinoza saw prophecy as a form of imagination, which did not, on its own, involve any certainty of truth, but required some extrinsic reason to assure its recipients of its objective reality.