Introduction Rabbi Eliezer Hisma was a student of Rabban Gamaliel and of Rabbi Joshua. Legend has it that he was a great mathematician.
Rabbi Eliezer Hisma said: the laws of mixed bird offerings and the key to the calculations of menstruation days these, these are the body of the halakhah. The calculation of the equinoxes and gematria are the desserts of wisdom. Rabbi Eliezer Hisma states that there are two different types of laws that are “the body of the halakhah”. This means that they are essential halakhot, paradigmatic of the Oral Torah. The “laws of mixed bird offerings” refers to cases where one type of bird offering accidentally became mixed up with a different type. For instance an obligatory offering became mixed up (perhaps in a net or in a dovecote) with a voluntary offering; or a sin offering became mixed up with a whole burnt offering. There is a whole tractate entitled Kinnim that deals with these situations. The second type of halakhah are the calculations of menstruation days. This refers to calculating how many days there are between a woman’s cycle. It is important to figure this out so that women can distinguish between menstrual bleeding and other types of bleeding. Much of tractate Niddah is devoted to this subject. There are two types of laws that are only “desserts of wisdom”. That is they help learning, but they are not essential. One is astronomy, that is the calculation of the seasons. The other is gematria, which refers to the adding up of numbers (obviously this is from the same word as geometry). According to the Meiri, Rabbi Eliezer Hisma wants to guide a person in his course of study. The first thing a person should learn is Torah. The two areas of Torah learning that he mentions are representative of some of the most difficult subjects in the learning of Torah. A person should first learn Torah so well, that he is able to master even tractates Kinnim and Niddah. Afterwards, when he has completely mastered even in these subjects, a person should turn his attention to science and mathematics, represented here by astronomy and geometry.