(The Mishnah taught that a Cheresh who can speak but not hear should not separate Terumah, but if he did, it is valid.) This was needed according to R. Yosi, who says (in a Mishnah in Maseches Berachos) that if a person recited the Shema without his ears hearing it, he has not fulfilled his obligation. (In contrast, such a Cheresh separating Terumah is valid even though he didn't hear his recital of the Beracha.)

(Mishnah in Maseches Megillah): All may read the Megillah, except a Cheresh, an insane person and a child.

(Rav Masnah): That Mishnah is according to R. Yosi.

(R. Yosi {the Amora}): Originally, we thought that R. Yosi and the Chachamim disagree over reciting the Shema, as the pasuk there says, "Hear'', but in all other Mitzvos, there is no such requirement.

But from the statement of Rav Masnah (above, about the Mishnah in Megillah) that it is according to R. Yosi, this shows that the disagreement between the Chachamim and R. Yosi must apply in all Mitzvos.

Question: What is R. Yosi's source for this requirement for all of the Mitzvos? The pasuk states (Shemos 15:26), "And listen to His Mitzvos'' - Make heard to your ears that which you mouth speaks.

(Rav Chisda): The Mishnah (in Megillah) should not teach the case of Cheresh (as a mute cannot read the Megillah) - it was written there merely because of the habit of Mishnayos of always writing together the three cases of Cheresh, an insane person and a child.

(R. Yosi {the Amora}): It is logical that Rav Chisda would agree that the Mishnah about Terumah is according to R. Yosi.

(R. Bun): The Mishnah itself shows that it was authored by R. Yosi, as the beginning of the Mishnah taught 'Five should not separate and if they did, it is not valid' and the list didn't include a Cheresh who cannot speak. And if you will suggest that it's because they are not valid, but for such a Cheresh it would be valid - why then is it absent from the later Mishnah that listed people who should not separate Terumah, but if they did, it is valid? Rather, the Mishnah follows R. Yosi (and the reason it wasn't recorded in the list of five is because it is the subject of a dispute).

(Mishnah in Maseches Chagigah): All are obligated to be seen in the Temple on the festivals (Pesach, Shavuos and Succos), except for a Cheresh, an insane person and a child.

(Chevraya citing R. Elazar): (A Cheresh is exempt because) the pasuk states (Devarim 31:12), "In order that you will hear and in order that you will learn''. This only excludes one who can speak but not hear. What about one who can hear but not speak?

(R. Hila citing R. Elazar): It's written "L'Ma'an Yilmedun'' ('in order that you will learn'), but it should be read "L'Ma'an Yilameidun'' ('in order that you will teach') (which requires the ability to speak).

(R. Yona): This means that the rule that Rebbi taught in the Mishnah (Chulin 48(c)) that 'the standard Cheresh of Chazal is one who cannot hear or speak' isn't always accurate, as we have found that one who can hear but not speak is called a Cheresh and one who can speak but not hear is also called a Cheresh.

Another example is the Mishnah in Yevamos - 'A Cheresh that performed Chalitzah, a Chareshes (female Cheresh) that performed Chalitzah and a woman who performed Chalitzah with a child - the Chalitzah is invalid'. And R. Yochanan said that this applies in a case when he or she couldn't say (the required declarations - as written in Devarim 25). This shows that even if someone can hear, they could still be called a Cheresh (if they cannot speak)?!

This is further support for R. Yona who said that Rebbi's rule isn't always accurate.