An example of a relationship that is forbidden by Rabbinic ordinance is a person’s son’s daughter-in-law. The Gemara suggests that one’s daughter’s daughter-in-law is also forbidden, mainly out of concern that the two cases will be confused with one another. This explanation came as a result of an enigmatic story related by Rav Hisda in the Gemara. Rav Hisda tells that in his youth he heard a great man – Rabbi Ami – teach, “a daughter-in-law is forbidden because of a daughter-in-law.” Uncertain of the meaning of this statement – and informed by the kalda’ei (Chaldean astrologers) that he would grow up to be a teacher – Rav Hisda decided that, if he turned out to be one of the Sages, he would ascertain the meaning on his own, and if he were to become a simple teacher of children, he would ask the Sages who he met in the synagogue. Now as an adult, Rav Hisda realized on his own that Rav Ami meant that a person’s daughter’s daughter-in-law is forbidden lest she be confused with a person’s son’s daughter-in-law.
The kalda’ei to which Rav Hisda referred appear to be the same as the kasda’ei, one of the names of the Babylonians. Nevertheless, already in the Book of Daniel (see, for example, 2:2) we find that the kasda’ei were people who engaged in a specific profession – they were stargazers, or astrologists.
Although the Sages actively discouraged people from turning to these astrologists in order to learn the future – because such behaviors appear to contradict the commandment of tamim tihiyeh im HaShem elokekha (see Devarim 18:13) – still, many people, and especially children and simple folk, would turn to them with questions about their future. Some rishonim understood that the Sages accepted that these kalda’ei actually had the ability to foresee the future on some level, but taught the people that they should not rely on them too much. The Rambam, however, understands that these fortunetellers are forbidden specifically because their alleged abilities are nothing but foolishness (see his Mishneh Torah, Hilkhot Avodat Kokhavim 11:16).