Bekhorot 23B

אחת זו ואחת זו עד לכלב, הא חזיא לכלב! קשיא.

Both this one and that one, i.e., items of both forms of impurity, impart impurity until they are no longer fit to be eaten by a dog, it poses a difficulty to the opinion of R' Yirmeya, as this mass is fit for a dog. The Talmud concludes: Indeed, it is difficult.

גופא, בר פדא אמר: טומאה חמורה עד לגר, וטומאה קלה עד לכלב. ורבי יוחנן אמר: אחת זו ואחת זו עד לכלב. מאי טעמא דבר פדא? דכתיב: ״לא תאכלו כל נבלה לגר וגו׳״, הראויה לגר קרויה ״נבילה״, ושאינה ראויה לגר אינה קרויה ״נבילה״.

The Talmud discusses the matter itself. Bar Padda says: An item with severe ritual impurity imparts impurity to substances until it is no longer fit to be eaten by a ger toshav, but an item with light impurity imparts impurity to substances until it is no longer fit to be eaten by a dog. And R' Yoḥanan says: Both this one and that one impart impurity until they are no longer fit to be eaten by a dog. The Talmud asks: What is the reason for the ruling of bar Padda? As it is written: “You should not eat of any animal carcass; you may give it to the ger who is within your gates” (Deuteronomy 14:21). Bar Padda understands from this verse that an item which is fit for a ger toshav is called an animal carcass, and that which is not fit for a ger toshav is not called an animal carcass.

ואידך? למעוטי היכא דהסריחה מעיקרא. ואידך? הסריחה מעיקרא לא צריכא קרא למעוטי, עפרא בעלמא הוא.

The Talmud asks: And with regard to the other Sage, R' Yoḥanan, how does he interpret that verse? He maintains that it serves to exclude a carcass that was decayed from the outset, e.g., if the flesh started to decompose while the animal was still alive, due to severe injury. But if the carcass was initially edible and later decayed, it continues to impart ritual impurity provided it is fit for a dog. The Talmud asks: And with regard to the other Sage, bar Padda, how does he respond to this claim? A verse is not necessary to exclude such a case where the carcass was decayed from the outset, as it is merely like dust and obviously does not impart impurity.