Nazir 64A

מאי טעמא דתנא קמא? אמר רבי יצחק בר אבודימי, כתיב: ״בכל השרץ השרץ״, כל מקום שהוא שורץ, וכתיב ״על הארץ״. הא כיצד? ודאי מגעו ⁠— טמא, ספק מגעו ⁠— טהור.

What is the reason of the first tanna for declaring that in all uncertain cases of floating impurity the person or item remains pure? R' Yitzḥak bar Avudimi says: In the passage dealing with the impurity of creeping animals and the prohibition of eating them it is written: “With any swarming thing that swarms” (Leviticus 11:43), indicating that a carcass of a creeping animal renders items impure in any place where it swarms. And it is written: “All swarming things that swarm upon the earth” (Leviticus 11:42), indicating that it transmits impurity only if it is on the earth. How so? How can one reconcile these two verses? Definite contact with it renders one impure; one who has uncertain contact with it, e.g., the impurity is floating, remains pure.

ורבי שמעון מאי טעמא? אמר עולא: כתיב ״אך מעין״, וכתיב ״יטמא״ (עד הערב), הא כיצד? צפה בכלים ⁠— טמא, בקרקע ⁠— טהור.

The Talmud asks: And R' Shimon, what is the reason for his opinion? Ulla said that as it is written: “Nevertheless a fountain or a cistern in which there is a gathering of water should be pure” (Leviticus 11:36), this indicates that a creeping animal found in one of these places does not impart impurity. And it is written in the same verse: “He who touches their carcass should be impure until evening,” which indicates that it does render one impure. How so? If the impurity was floating in water contained in vessels, the item it touched is rendered impure, but if the water was in the ground itself, e.g., in a spring or pit, the item it touched is pure.