The Shach asks a question on the Rema in the Shulchan Aruch . The Gra answers it in one way and Rav Shmuel Rozovski of Ponovitch answers it differently based on a Tosphot in Ketuboth page 20. The subject in short is this. The Shulchan Aruch and Rema write that witnesses need to testify verbally, not by writing. But to validate a document, the Rema says one can testify by writing. And yet later on when the Shulchan Aruch writes if one is a witness, and then became unable to speak , he cannot testify as for the validity of his signature (even by writing). And the Rema does not disagree there. Why? The answer of the Gra is short, but Rav Shach explains it thus. The Rema holds like the Rivash [Rav Izhak ben Sheshet] that a witness can validate his signature by writing. This is coming like Rav Kahana in Gitin 71 side a who says a deaf person can write, ‘’Write and give a divorce to my wife, and it is valid. But a person who cannot speak cannot give testimony because not being able to speak is a defect of the body. So, to testify as for the validity of one’s signature we need valid witnesses, and if we have that then they can write that the signature is valid. But one who cannot speak is not a valid witness at all.] The problem I see is that Rav Kahana says nothing about validating one’s signature. He only speaks about a deaf person giving a command in writing to divorce his wife. I might mention that even though R. Yochanan disagrees with Rav Kahana still there is R. Isaac on page 71 side b who explains the Mishna there to be in accord with Rav Kahana. {So we can see why the Rivash [Issac ben Sheshet] decided like Rav Kahana against R. Yochanan.} ----------------------------------------------------------------------------------------------------------The ש''ך asks a question on the רמ''א in the שלחן ערוך. The גר''א answers it in one way and רב שמואל רוזובסקי answers it differently based on a תוספות בכתובות דף כ'. The subject in short is this. The שלחן ערוך and רמ''א write that witnesses need to testify verbally, not by writing. But to validate a document, the רמ''א says one can testify by writing. And yet later on when the שלחן ערוך סימן מ''ו ס''ק ל''ו writes if one is a witness, and then became unable to speak, he cannot testify as for the validity of his signature (even by writing). And the רמ''א does not disagree there. Why? The answer of the גר''א is short, but רב שך explains it thus. The רמ''א holds like the (ריב’’ש) רב יצחק בן ששת that a witness can validate his signature by writing. This is coming like רב כהנא in גיטין דף ע''א side a who says a deaf person can write, ‘’Write and give a divorce to my wife, and it is valid. But a person who cannot speak cannot give testimony because not being able to speak is a defect of the body. So, to testify as for the validity of one’s signature we need valid witnesses, and if we have that, then they can write that the signature is valid. But one who cannot speak is not a valid witness at all.] The problem I see is that רב כהנא says nothing about validating one’s signature. He only speaks about a deaf person giving a command in writing to divorce his wife. I might mention that even though ר’ יוחנן disagrees with רב כהנא still there is ר' יצחק on דף ע''א who explains the משנה there to be in accord with רב כהנא. {So, we can see why the ריב''ש יצחק בן ששת decided like רב כהנא against ר’ יוחנן.}