Rashi (6b D”H kma”l) explains that the term ‘reshus hayachid gemura’ indicates that according to the Rabbanan a Reshus Hayachid requires to be completely enclosed from all sides. Rashi in Eruvin (11b D”H lizrok) says that a Reshus Hayachid requires only three walls. How can we resolve both Rashi’s?
Rashi says that mei raglayim in considered tamei even if there are two reasons for safek 1) maybe it’s from an animal 2) Maybe from a person who is tahor. Generaly we know that a double safek (sfek sfeka) is permitted – why is this case different?