Whether he be a sinner or no, I know not. — The words, “or no,” are added to the text, but rightly complete the meaning. He, like his parents, will confine himself to matters of fact coming under his own certain knowledge. They had declared authoritatively that they knew this Man to be one whose life was characterised by sin. He is convinced that this cannot be so (John 9:31; John 9:33), but he does not dispute their assertion; he simply makes his own, which cannot be gainsaid, and which cuts the ground from under them.
One thing I know. — For this use of “one thing” to mark the chief thing which is so important that all others are excluded, and it is left as the only one in the mind, comp. Mark 10:21 (“one thing thou lackest”) and Luke 10:42 (“one thing is needful”).
Whereas I was blind, now I see. — Better, Being a blind man, now I see. He places the two things in contrast. He was the well-known “blind man,” whose experience of his own blindness had extended from birth to manhood. They declare that he has not been healed. He is conscious of his power to see, and this one thing he affirms. The difficulty is of their making; let them explain it as they think best.