Metaphorical. He was probably referring to Arima's peripheral vision which is more pronounced when wearing glasses. It's somewhat similar to how Kaneki tried aiming for Shachi's eyes but embarrisingly failed
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He also has a habit of using dramatic metaphorical words.
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On the other end, Arima's worn glasses
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so his myopia or whatever condition he has could deteriorate enough for Arima to develop a lethal blind spot (tunnel vision).
Glaucoma and Retinitis Pigmentosa for example:
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Either way, Arima didn't even struggle to kill him. There's more to Arima than meets the eye.
People have been saying that especially his right eye has bad vision. Apparently Kaneki scratched him on his right side when they fought in P1 and they theorize it's because he legitimately didn't see the attack coming with his right eye. Don't know what to make of that.
I think Shachi was referring to Arima's eyes being lifeless tho.
I'd say that Shachi was talking about the look in his eyes. Arima comes as off he doesn't really care if he wins or loses, and we know that he doesn't have any loved ones to speak of. (As he never writes a letter.) So it may be that Shachi is referring to that as the majority of this chapter is how far Shachi went to protect his "child" and probably credit that to his own strength. Arima lacks that, and that is his "blinds spot", he lack reason and will to fight. He simply does it because he has the ability and probably feels that it is the right thing to do.
Which may be how he dies, to Ken. Seeing Ken as somewhat if a child to protect.
I'd say that Shachi was talking about the look in his eyes. Arima comes as off he doesn't really care if he wins or loses, and we know that he doesn't have any loved ones to speak of. (As he never writes a letter.) So it may be that Shachi is referring to that as the majority of this chapter is how far Shachi went to protect his "child" and probably credit that to his own strength. Arima lacks that, and that is his "blinds spot", he lack reason and will to fight. He simply does it because he has the ability and probably feels that it is the right thing to do.
Which may be how he dies, to Ken. Seeing Ken as somewhat if a child to protect.