Zevachim 16B

בעל מום, שאוכל ⁠– אם עבד חילל; אונן, שאין אוכל ⁠– אינו דין שאם עבד חילל?!

with regard to a blemished priest, who may partake of the meat of offerings, if he performed sacrificial rites he has desecrated the service, then with regard to an acute mourner, who may not partake of the meat of offerings, is it not right that if he performed sacrificial rites he has desecrated the service?

מה לבעל מום, שכן עשה בו קריבין כמקריבין!

The Talmud rejects the inference: One cannot derive this halakha from the case of a blemished priest, as what is notable about a blemished priest? It is notable in that the Torah rendered animals that are sacrificed like the priests who sacrifice them. Since this stringency does not exist with regard to acute mourning, perhaps an acute mourner may perform sacrificial rites as well.

זר יוכיח. מה לזר, שכן אין לו תקנה!

The Talmud responds: The case of a non-priest will prove that this is no reason to reject the a fortiori inference, since there is no equivalence between priest and sacrifice in this regard, yet a non-priest desecrates the service. The Talmud rejects this as well: One cannot cite a proof from the case of a non-priest, as what is notable about the case of a non-priest? It is notable in that he has no remedy, i.e., a non-priest may never perform the sacrificial rites. By contrast, an acute mourner will eventually become permitted to perform the service.

בעל מום יוכיח. וחזר הדין. לא ראי זה כראי זה, ולא ראי זה כראי זה; הצד השוה שבהן ⁠– שהן מוזהרין, ואם עבדו חיללו; אף אני אביא אונן ⁠– שמוזהר, ואם עבד חילל.

The Talmud responds: A blemished priest will prove that this is no reason to reject the inference, as even though the priest may perform the service if the blemish is healed, he desecrates the service so long as he remains blemished. And the inference has reverted to its starting point. Therefore, one learns the halakha from the combination of the cases of a blemished priest and a non-priest: The aspect of this case is not like the aspect of that case, and the aspect of that case is not like the aspect of this case. Their common element is that they are prohibited from performing sacrificial rites, and if they performed such rites they have desecrated the service. Therefore, I will also include an acute mourner, who is prohibited from performing sacrificial rites, and conclude that if he performed sacrificial rites he has desecrated the service.