Haï treated of the civil law of the Talmud, of contracts, loans, boundaries and oaths, with systematic precision. He did this as no one before him had done, and he therefore became the model and authority for later generations. He did not enter upon the field of metaphysics, but although he was not a philosopher, he had sound opinions on mysticism. Surrounded with a halo of religion, a mystic belief often appears reasonable to those of weak reasoning powers, but Haï perceived its deceptive character. The belief in miracles has, in every country, at all times, and in all creeds, befogged the intellect of unthinking men, and robbed them of the ability to form a rational view of divine wisdom and of life. This belief was fostered by the Jews in many ways, and took as firm a hold on them, as it had on the Christian and the Mahometan world. It was especially prevalent in Palestine and Italy. Its devotees believed that any one who is truly pious can perform at will miracles as great and surprising as those of the prophets of old. They thought, however, that for this purpose it is necessary to pronounce certain magical formulæ, consisting of various combinations of the letters in the name of God. Haï's true religious insight prompted him to write indignantly against this belief, which, despite the fact that his father was not free from it, he considered a desecration of religion. A pupil of Jacob ben Nissim of Kairuan once asked Haï what he thought of the magical power of the names of God, which, many boasted, they could use. Haï answered briefly and sensibly:--"If any one by the mere use of formulæ could perform miracles, and thereby alter the course of nature, wherein lay the distinction of the prophets?" God gave the prophets the power of temporarily altering the laws of nature that they might prove themselves His true messengers. Now, if pious persons could do the same, and if there happened to be many of them, miracles would become daily occurrences, and the motion of the sun from west to east would appear no more extraordinary than its common motion in the opposite direction--in short, miracles would cease to be miracles. "It is wrong," said Haï, "to make use of the name of God for such purposes," and he warned the people against this practice, in which there is much doubt and little truth; and a man must be indeed foolish who believes everything.