270. Wait Outside: The prohibition against the Kohein Gadol going under the same roof as a corpse
He shall not go near any dead person… (Leviticus 21:11)
We said previously, in Mitzvah #263, that a regular kohein could not become impure for a dead person except for his seven closest relatives: father, mother, brother, unmarried sister, son, daughter and wife. Just as the kohein has a higher level of personal sanctity than other Jews, the Kohein Gadol (High Priest) has a higher level still. He may not ritually defile himself for any person, not even his closest family members. In this mitzvah, he is specifically prohibited from entering a building where a corpse is lying in repose.
The reason for this mitzvah, exceeding even that of regular kohanim, is that the Kohein Gadol is always “on call.” He must maintain a constant state of purity so that he will be ready to serve at the drop of a turban. Special consideration is given to regular kohanim to grieve for their relatives, but the Kohein Gadol, by virtue of his position, must overcome the usual, normal human instincts. In a sense, the Kohein Gadol is expected to dwell in a spiritual world, leaving temporal matters behind.
This prohibition applies when there is a Temple (and, therefore, a Kohein Gadol). It applies to the High Priest even if he’s traveling abroad and nowhere near the Temple. This mitzvah is discussed in the Talmud in tractate Nazir (42b-44a, 46b-49b) and codified in in the the Mishneh Torah in the seventh chapter of Hilchos Aveil. It is #167 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.