Chapter 3:5 ג׳:ה׳
1 א

A scroll on which the writing has become erased and eighty-five letters remain, as many as are in the section beginning, "And it came to pass when the ark set forward" (Numbers 10:35-36) defiles the hands. A single sheet on which there are written eighty-five letters, as many as are in the section beginning, "And it came to pass when the ark set forward", defiles the hands.
All the Holy Scriptures defile the hands.
The Song of Songs and Kohelet ( defile the hands.
Rabbi Judah says: the Song of Songs defiles the hands, but there is a dispute about Kohelet.
Rabbi Yose says: Kohelet does not defile the hands, but there is a dispute about the Song of Songs.
Rabbi Shimon says: [the ruling about] Kohelet is one of the leniencies of Bet Shammai and one of the stringencies of Bet Hillel.
Rabbi Shimon ben Azzai said: I have received a tradition from the seventy-two elders on the day when they appointed Rabbi Elazar ben Azariah head of the academy that the Song of Songs and Kohelet defile the hands.
Rabbi Akiba said: Far be it! No man in Israel disputed that the Song of Songs [saying] that it does not defile the hands. 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. If they had a dispute, they had a dispute only about Kohelet.
Rabbi Yohanan ben Joshua the son of the father-in-law of Rabbi Akiva said in accordance with the words of Ben Azzai: so they disputed and so they reached a decision.

Today's mishnah contains a fascinating argument over whether two books from the Tanakh, Kohelet (Ecclesiastes) and the Song of Songs (Shir Hashirim) defile the hands. This is basically an argument over whether these books should be included in the Biblical canon. We cannot be certain why there was an argument over these specific books. Probably the content of these books caused certain sages to wish to cut them out of the canon. Kohelet is a deeply skeptical work, one which frequently questions whether the world works justly. One central message of the book is that the righteous and the evil receive the same lot in this world. Sometimes life is even worse for the righteous than it is for the evil. Song of Songs seems to be a love song between a man and a woman. What place does such literature have, some rabbis asked, in the biblical canon? In the end, both books were accepted into the canon and have been found in Jewish (and non-Jewish) bibles ever since.
Section one: The smallest "parsha" in the Torah is Numbers 11:35-36. These two verses, which contain 85 letters, are set off as a "parsha" in the traditional writing of the Torah. The mishnah uses this number as a paradigm for what constitutes a "scroll" such that it would defile the hands. There either have to remain 85 letters from a scroll that used to have more, or a new scroll has to already have 85 letters. Less than 85 letters and the scroll will not defile the hands.
Sections 2-9: The remainder of the mishnah is an extended discussion/argument over whether two books, Kohelet and Shir Hashirim (Ecclesiastes and Song of Songs) defile the hands (i.e. are they part of the canon).
Ultimately, we know that both of these books were accepted into the Jewish Bible, and indeed were rich sources for rabbinic midrashim. There are probably several reasons why they were accepted. First and foremost, Shir Hashirim is explicitly attributed to King Solomon and Kohelet is ascribed to a king in Jerusalem, traditionally understood as King Solomon. While modern scholars do not accept the historical accuracy of these ascriptions, rabbis certainly didn't doubt them. Being ascribed to an ancient king certainly helps if you want to be part of the Bible!
Secondly, Shir Hashirim merited a metaphorical interpretation. The love story was not between a man and a woman but rather between the people of Israel and God. It is this metaphorical, mystical and at times erotic poetry between God and Israel that causes Rabbi Akiva to call it the holiest book.
And while Kohelet does express deep skepticism concerning justice in the world, it ultimately ends with the famous verse, "The sum of the matter, when all is said and done: Revere God and observe His commandments! For this applies to all mankind." The rabbis too certainly would have ascribed to this directive.