If I plead against God mine own righteousness and innocency, God is so infinitely wise and just, that he will find sufficient matter of condemnation from my own words, though spoken with all possible care and circumspection; or he will discover so much wickedness in me of which I was not aware, that I shall be forced to join with him in condemning myself. If I say, I am perfect; if I were perfect in my own opinion; if I thought myself completely righteous and faultless, it, i.e. my own mouth, as he now said, or, he, i.e. God, who is easily understood by comparing this with the former verses, where the same he is oft mentioned, shall also prove me perverse.
Related Commentaries of Job 9:20
Job 9:20
20 If I justify myself, mine own mouth shall condemn me: if I say, I am perfect, it shall also prove me perverse.