פירוש על קידושין 82:8

We have discussed four issues—marriage, divorce, terumah and sacrifices, and stated that the right to appoint an agent cannot be derived from one of them and therefore the Torah had to teach agency in all of them. But maybe we can derive from two of them that agency works in the others? If so, why would the Torah have to teach the efficacy of agency in the others? The Talmud will now test this out.
We could not derive agency in sacrifices from the other cases, because sacrifices are holy whereas the other matters are either not holy at all, or less so. Therefore, the Torah had to teach the efficacy of agency with regard to sacrifices.

We could not derive agency in divorce from agency sacrifices and terumah because sacrifices and terumah can be designated by thought only. This is not true of divorce—the man must give the woman a get. It is not sufficient for him to merely think that he wishes to divorce her.