aninus and aveilus (I)

Those learning the daf this past Shabbos (Zevachim 1010) got a taste of the sugya of aninus. The d'oraysa halachos of aninus can basically be summarized on one sentence: a person who is an onein is not allowed to eat kodshim or ma'aser sheni. The gemara learns from the pasuk, "V'achalti chatas ha'yom?" that Aharon Hakohen could not eat kodshim after the deaths of Nadav v'Avihu. (I guess on a d'oraysa level that we can also throw in the exemption from positive mitzvos as well if it is based on oseik b'miztvah patur min hamitzvah.)

The halacha of aninus is completely different than that of aveilus. Aninus is d'oraysa; aveilus according to most Rishonim is derabbanan. An aveil is allowed to eat kodshim; an onein is not. Aninus begins when a person finds out that his relative has passed away and ends with burial; aveilus begins at burial and extends through the shiva period. An aveil doesn't wear shoes, sit on a comfortable chair, launder his clothes, etc., none of which is practiced during aninus.

The Rambam opens hilchos aveilus by saying that observing the first day of aveilus -- the day of death and burial -- is d'oraysa, based on this same pasuk of "V'achalti chatas ha'yom" that the gemara learns the issur of achilas kodshim for an onein from. The Rishonim and Achronim all ask how the Rambam could mix together these two very different sets of halachos. When a person is an onein he is not an aveil; when he is an aveil he is not an onein -- never the twain shall meet. To be continued bli neder...