Why does the mishna assume blind parents can't identify their son?

Question

Sanhedrin 8:4 lists a bunch of things that can prevent parents from declaring their son a ben sorer umoreh, a stubborn and rebellious son. These include lame parents (because they cannot bring him out), those without hands (because they cannot take him), and those who are mute (because they can't speak the declaration).

Those all (mostly) make sense to me, but the mishna also excludes a parent who is blind because such a person can't say "this our son". I don't understand this one. The expanded translation at Sefaria (for which I don't know the source) says it's because they cannot point to the boy, but why would they need to point? They're holding onto him, and that they hold him is the reason to exclude someone missing a hand. A note in the Soncino translation says that "this our son" means they need to see him, but I found no further explanation in the g'mara or in the Soncino notes. Why do they need to see him if they're holding onto him and his identity is not in question?

What is it about being blind that prevents parents from identifying their son?

I realize that our tradition places a lot of barriers here intentionally and that the case is theoretical (tradition says there never was a ben sorer umoreh). I'd still like to understand the reasoning for this one of these many barriers.

Answer 1

Rashi in several places comments that זה means that you can see and point at something and say, “This is it!”

For instance, on Shemos 12:2:

הזה. נִתְקַשָּׁה מֹשֶׁה עַל מוֹלַד הַלְּבָנָה, בְּאֵיזוֹ שִׁעוּר תֵּרָאֶה וְתִהְיֶה רְאוּיָה לְקַדֵּשׁ, וְהֶרְאָה לוֹ בְּאֶצְבַּע אֶת הַלְּבָנָה בָּרָקִיעַ וְאָמַר לוֹ כָּזֶה רְאֵה וְקַדֵּשׁ (שם).‏

”This” - Moshe was struggling with the birth of the moon - in what amount should it be seen and be fit for sanctification? He showed him with a finger the moon in the sky and said, “Like this should you see and sanctify.”

Shemos 15:2:

זה א-לי. בִּכְבוֹדוֹ נִגְלָה עֲלֵיהֶם וְהָיוּ מַרְאִין אוֹתוֹ בְּאֶצְבַּע, רָאֲתָה שִׁפְחָה עַל הַיָּם מַה שֶּׁלֹּא רָאוּ נְבִיאִים (מכילתא):‏

”This is my G-d” - In His honor He revealed upon them, and they saw him with a finger. A maidservant saw on the sea what the prophets didn’t see.

Shemos 32:1:

כי זה משה האיש. כְּמִין דְּמוּת מֹשֶׁה הֶרְאָה לָהֶם הַשָּׂטָן, שֶׁנּוֹשְׂאִים אוֹתוֹ בַּאֲוִיר רְקִיעַ הַשָּׁמָיִם (שבת פ"ט):‏

“For this man Moshe” - Like the appearance of Moshe the Satan showed them, that he held him in the air of the sky of heaven.

And so on and so forth.

So, too, by בננו זה: the parents have to see the child and point to him to be able to say “this.” As Rashi says succinctly on Sanhedrin 45b:

בננו זה - משמע שרואין אותו

”Our son, this one” - implies that they see him.

Answer 2

I think that what you wrote in the question "I realize that our tradition places a lot of barriers here intentionally..." is the main reason for that Drasha (study).

Do you really understand why lame/mute/amputee parent has a limited authority over his son?

I think it's obvious from more examples:

סנהדרין סט ע"ב ע ע"א

תניא אמר רבי שמעון בדין הוא שתהא בת ראויה להיות כבן סורר ומורה שהכל מצויין אצלה בעבירה אלא גזירת הכתוב היא בן ולא בת

Sanhedrin 69b-70a

We learned: Rabbi Shimon says, rationally, a girl could also become Ben Sorer Umore, because many are found with her (can sin with her), but the Tora says, boy and not girl.

We know that usually, men and women are equal where it comes to Dinim, why not here?

סנהדרין עא ע"א

...רבי יהודה אומר אם לא היתה אמו שוה לאביו בקול ובמראה ובקומה

Sanhedrin 71a

Rabbi Yehuda says: if his mother has no identical voice, looks, and height to his father (their son can't be Ben Sorer Umore)

Identical height (very rare), voice (almost impossible) and looks (impossible)?! it's not me who says it's impossible, that's how the Gemara explains R' Yehuda opinion!

Does it makes any sense? only clone, lab-generated parents has authority for Ben Sorer Umore?

R' Yehuda Drasha is very weak. He learns this from a word that is used for other Drasha, but still, as you said, our tradition places a lot of barriers here intentionally, there is no other sense in it.