Apparently, during Temple times, the priests in charge of holy books took advantage of the fact that priests avoided touching anything impure while they were carrying food, lest the food become impure. By declaring that holy books “caused hands to become impure,” priests would avoid eating or bringing food into the rooms with the holy books. This, of course, kept them observing the librarian’s number one rule: No eating in the stacks!
This explanation basically follows R. Mesharsheya in the Babylonian Talmud (Shabbat 14a). Mishnah Yadayim 4:6 reports the Sadducees (a sect from Second Temple times) crying out against the Pharisees (another sect from whom the rabbinic movement evolved) for saying that “Holy Scriptures make the hands impure, but the books of Homer don’t make the hands impure.” If R. Mesharsheya’s assumption about concern for damage to the books is correct, then apparently, the Pharisees didn’t care if worms and mice ate copies of Homer.
Since the language “cause hands to become impure” was associated with holy books, the rabbis after the destruction of the Temple in 70 CE continued to use the phrase to describe which books were holy and conversely which books were not holy (the “not holy” books do not make hands impure). The discussion below questions whether the Song of Songs and Ecclesiastes belong in the Bible, or more precisely, it questions whether there were ever any questions about whether they belong in the Bible.
Mishnah Yadayim 3:5.
All the holy writings make the hands impure. The Song of Songs and Ecclesiastes make the hands impure.
R. Judah says: The Song of Songs makes the hands impure, but there is a dispute about Ecclesiastes.
R. Jose says: Ecclesiastes does not make the hands impure, but there is a dispute about the Song of Songs.
R. Simeon says: Ecclesiastes is one of the leniencies of Bet Shammai [who say it does not make the hands impure] and one of the stringencies of Bet Hillel [who say it does make the hands impure]..
R. Simeon b. Azzai said: I received a tradition from the seventy-two elders on the day when they appointed R. Eleazar b. Azariah head of the court that the Song of Songs and Ecclesiastes make the hands impure.
R. Akiba said: God forbid! No one in Israel ever disagreed about the Song of Songs [by saying] that it does not make the hands impure. For the whole world is not as worthy as the day on which the Song of Songs was given to Israel; for all the writings are holy but the Song of Songs is the holy of holies. So if they had a dispute, they had a dispute only about Ecclesiastes.
Johanan b. Joshua the son of the father-in-law of R. Akiba said: in accordance with the words of Ben Azzai so they disputed, and so they reached a decision.
Pronounced: MISH-nuh, Origin: Hebrew, code of Jewish law compiled in the first centuries of the Common Era. Together with the Gemara, it makes up the Talmud.