Study Guide Zevachim 16

The Gemara presents three proofs that the service of a non-priest (zar) in the Temple is invalid: one from a verse, and two derived through a kal va-chomer argument. It then brings four proofs that the service of a mourner, before burial (onen), in the Temple is also invalid: two from verses and two from kal va-chomer reasoning.

Rava attempts to limit the disqualification of the onen to the case of an individual offering, based on a kal va-chomer from ritual impurity that is permitted in communal offerings. In other words, if impurity does not invalidate a communal offering (when the majority of the community is impure), perhaps mourning should not invalidate it either. However, Rava bar Ahilai rejects this argument, claiming that accepting such a kal va-chomer would open the door to additional a fortiori arguments that could lead to incorrect halakhic conclusions, and therefore it should not be accepted.