Bekhorot 33A

ובית הלל: הני מילי תם, אבל בעל מום כתיב ״הטמא והטהור יחדיו יאכלנו״, ומה טמא שאינו אוכל בקדשים קלים אוכל בבכור, זר שאוכל בקדשים קלים אינו דין שיאכל בבכור.

And Beit Hillel would respond to that claim: This statement applies only to an unblemished firstborn offering, which is sacrificed upon the altar. But concerning a blemished firstborn it is written: “You should eat it within your gates; the impure and the pure may eat it alike” (Deuteronomy 12:22; see Deuteronomy 15:22). Beit Hillel derive an a fortiori inference: And if a ritually impure priest, who may not eat the meat of offerings of lesser sanctity, nevertheless may eat the meat of a blemished firstborn offering, then with regard to a non-priest, who may eat the meat of offerings of lesser sanctity, e.g., peace offerings and animal tithe offerings, is it not logical that he may eat the meat of a blemished firstborn offering?

איכא למיפרך: מה לטמא, שכן הותר מכללו בעבודת צבור!

The Talmud raises a difficulty: This a fortiori inference can be refuted: What is notable about the case of a ritually impure priest? It is notable in that its general prohibition was permitted in certain circumstances, specifically with regard to the communal service. If there are no ritually pure priests, the sacrificial service may be performed by ritually impure priests. By contrast, it is never permitted for a non-priest to perform the sacrificial service. Accordingly, one cannot derive the halakha concerning a non-priest via an a fortiori inference from an impure priest.

ובית הלל, אטו בעבודה קאמר? באכילה קאמרינן, אכילת זר עדיף.

And Beit Hillel would respond: Is that to say that the a fortiori inference was stated with regard to the sacrificial service, which was the subject of that refutation? Not so; rather, we state it with regard to the eating of sacrificial meat, and the eating of a non-priest is superior to that of a ritually impure priest. Accordingly, the refutation of the a fortiori inference is inapplicable.