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.