A.T. Robertson Commentary John 9:20

"His parents answered and said, We know that this is our son, and that he was born blind:" — John 9:20 (ASV)

We know that this is our son, and that he was born blind (Οιδαμεν οτ ουτος εστιν ο υιος ημων κα οτ τυφλος εγεννηθη). These two questions the parents answer clearly and thus cut the ground from under the disbelief of these Pharisees as to the fact of the cure (verse 18). So these Pharisees made a failure here.