Could do nothing - Could do no such work as this. This reasoning was conclusive. The fact that Jesus could perform miracles like this was full proof that he was commissioned by God - proof that never has been and never can be refuted. One such miracle proves that he was from God. But Jesus gave many similar proofs, and thus put his divine mission beyond the possibility of doubt.
Related Commentaries of John 9:33
John 9:33
33 If this man were not of God, he could do nothing.