Rashi explains that this botzek is called "cheresh" because its appearance does not tell us if it is chometz or not. It is similar to the ears of a deaf person, which, by merely looking at it does not tell us if it can hear or not.
well...
Shouldn't have Rashi said that it is similar to the mouth of a Cheresh, which, by merely looking at it does not tell us if it can talk or not?!.
The Bartenurah and the Rambam seem to agree with what I say.
Best wishes
Alex Lebovits, Toronto
Although many Rishonim argue on Rashi's explanation of Batzek Cheiresh as you noted, I am unsure if this is the reason that they disagreed with Rashi. Rashi definitely did not say "by merely looking at it one cannot tell if it can hear or not."
Although Rashi's phraseology here is difficult to translate, I understand that Rashi said "it is not apparent whether or not he can hear, as a person cannot understand the matter regarding the clarity and understanding."
Rashi is possibly saying that one cannot clearly define how much a person hears and understands via the person's usage of the ears, as a person's ears don't (naturally) move or show any indication of being used. This is unlike a mouth, which is clearly being used during speech. Although one might say a nose is similar, it makes more sense for the Gemara to use the example of a Cheiresh, as it is a more common disability.
All the best,
Yaakov Montrose