17 He said to them, “Are you so foolish? Don’t you understand that whatever goes into a person from outside cannot defile him? 18 For it does not enter his heart but his stomach, and then goes out into the sewer.” (This means all foods are clean.) 19 He said, “What comes out of a person defiles him.

We learned in the mishna that when the Sages went up to the upper story of the house of Ḥananya ben Ḥizkiya ben Garon, they were counted and issued eighteen decrees in accordance with the opinion of Beit Shammai. The Gemara asks: What are those eighteen matters? The Gemara answers: As we learned in a mishna, a list of the decrees that the Sages issued with regard to items whose level of impurity is such that if they come into contact with teruma they disqualify it. By means of that contact, the teruma itself becomes impure, but it does not transmit impurity to other items. These disqualify teruma: One who eats food with first degree ritual impurity status acquired as a result of contact with a primary source of ritual impurity, e.g., a creeping animal; and one who eats food with second degree ritual impurity status acquired as a result of contact with an item with first degree ritual impurity status; and one who drinks impure liquids of any degree of impurity; and one whose head and most of his body come into drawn water after he immersed himself in a ritual bath to purify himself; and a ritually pure person that three log of drawn water fell on his head and most of his body; and a Torah scroll; and the hands of any person who did not purify himself for the purpose of handling teruma; and one who immersed himself during the day, i.e., one who was impure and immersed himself, and until evening he is not considered completely pure; and foods and vessels that became impure by coming into contact with impure liquids. Contact with any of these disqualifies the teruma. The Gemara seeks to clarify these matters.