Mishnah 4 of the first chapter of Maseches Shabbos. This mishnah deals with two of the decrees mentioned in the previous mishnah - the things one may not do by the light of a lamp on Shabbos: not to pick through a garment for lice and not to read. The mishnah places these halachos in their historical setting: the visit of the Sages to the attic of Chananyah ben Chizkiyah ben Guryon.
Who was Chananyah ben Chizkiyah ben Guryon?
The Gemara explains that in the book of Yechezkel there were several matters that appeared to contradict the words of the Chumash. The Sages of that generation wished to suppress the book of Yechezkel, out of concern that people would not understand these matters and would come to err in their understanding of the Chumash itself. Chananyah ben Chizkiyah ben Guryon rose to the occasion, went up to an attic and secluded himself there for a long period, occupying himself with explaining the book of Yechezkel and reconciling it with the words of the Chumash.
The Sages would go up to visit him, and on one of those visits Beis Hillel and Beis Shammai had the opportunity to debate halachah. In practice Beis Hillel lost the vote, and in many halachos the votes went to Beis Shammai. At that gathering eighteen different decrees were enacted, and it is these that our mishnah discusses.
The words of the mishnah:
"Ve'eilu min hahalachos she'amru ba'aliyas Chananyah ben Chizkiyah ben Guryon" - these two halachos, not to pick through lice and not to read by the light of a lamp on Shabbos, are two of the halachos that were stated, and in fact decreed, in the attic where Chananyah ben Chizkiyah ben Guryon was staying.
"Kesh'alu levakro" - when the Sages went up to visit him in that attic.
"Nimnu verabu Beis Shammai al Beis Hillel" - they took a count and held a vote, and the votes favoring the position of Beis Shammai outnumbered those of Beis Hillel.
"Ushemonah asar devarim gazru bo bayom" - on that very day eighteen matters were decreed.
What were those decrees?
Two of them are the halachos mentioned above regarding the light of a lamp; nine of them deal with things that render a person tamei with regard to Terumah, so that he may not eat it; and several more besides.
The nine things that disqualify a person from eating Terumah:
One who eats food that is a rishon letumah (first degree of tumah).
One who eats food that is a sheni letumah (second degree of tumah).
One who drinks liquids that are tamei.
One who immersed in a mikvah and afterward put his head and the greater part of his body into drawn water.
One who did not even immerse recently but was tahor, and three lugin of drawn water were poured over him.
The books of Tanach - if they came into contact with Terumah, they disqualify it.
A person's hands that he did not wash - they disqualify Terumah by touching it.
Foods that became tamei through liquids, which liquids themselves became tamei through unwashed hands - these foods too disqualify Terumah.
Vessels that became tamei through liquids, which liquids became tamei by contact. Even though the one who touched them made the liquids a rishon letumah, and ordinarily a rishon letumah does not render vessels tamei, here they decreed that the vessel is tamei with regard to contact with Terumah.
This is not the place to elaborate on the reasons behind each one of these decrees, but each one has a reason of its own.
The remaining decrees:
The tenth: the daughters of the Kutim are tamei from their cradle, from the moment of their birth, on account of niddah.
The eleventh: anything whose circumference is a tefach transmits tumah in a tent, even though its width is not a tefach and only its circumference is.
The twelfth: one who harvests grapes in order to place them in the winepress - they have become susceptible to receiving tumah, even though the juice on them is destined to be lost.
The thirteenth: one who plants Terumah and it grows - even with a plant whose seed disintegrates, the growths are considered Terumah.
The fourteenth: one who was traveling on the road on Erev Shabbos and Shabbos began - it is preferable that he hand his purse to a non-Jew to carry it, rather than carry it himself in increments of less than four amos, a form of carrying that is not considered a desecration of Shabbos by Torah law.
The fifteenth and sixteenth: the two halachos in the previous mishnah - not to pick through lice and not to read by the light of a lamp.
The seventeenth: the bread, oil, wine and daughters of non-Jews are forbidden; all of these are counted as a single decree.
The eighteenth: that a non-Jewish child should have the status of a zav, so that Jewish children should not come to excessive closeness with non-Jewish children, which would lead to immoral behavior.
In summary: in this mishnah we learned that the two halachos taught earlier regarding the light of a lamp were stated in the attic of Chananyah ben Chizkiyah ben Guryon, who was occupied with reconciling the book of Yechezkel with the Chumash. During the Sages' visit to him they took a count and Beis Shammai outnumbered Beis Hillel, and on that day eighteen matters were decreed - nine of them concerning the disqualification of a person from eating Terumah, and the rest concerning tumah, susceptibility to receiving tumah, Shabbos and the laws of keeping distance from non-Jews.