Job 9:20

If I justify myself
Seek for justification by his own righteousness, trust in himself that he was righteous, say that he was so, and pronounce himself a righteous man, what would it signify?

mine own mouth shall condemn me;
the words of it being sinful, vain, idle, and frothy; and if a man is to be justified, and condemned by his words, he may be sure of the latter: indeed, "if any man offend not in word, the same is a perfect man", ( James 3:2 ) ; but let a man be as careful as he can, and keep ever such a guard upon his lips, such is the imperfection of human nature, that, though a Moses, he will speak unadvisedly with his lips, at one time or another, and in many things will offend; which would be his condemnation, if there was no other way to secure from it; nay, for a sinful man to justify himself, or to say that he is a righteous man by his own righteousness, and insist upon this before God, if he is tried upon it he must be condemned; yea, saying he is so is a falsehood, abominable to God, and enough to condemn him; and besides, a man that knows himself, as Job did, must be conscious of much sin within him, however externally righteous he may be before men; so that, should he say he was righteous, his conscience would speak, or cause his mouth to speak and contradict and condemn him:

[if I say], I [am] perfect;
not in an evangelical sense, as he was; but in a legal sense, so as to be free from sin, which no man that is perfect in a Gospel sense is; as Noah, Jacob, David, and others, who were so, yet not without sin; if therefore a man should assert this, he would not say that which was right, but what was perverse, as might be proved:

it shall also prove me perverse;
to be a wicked man; either he, God, shall prove, or it, his mouth, as in the preceding clause; for to say this is to tell a lie, which to do is perverseness, see ( 1 John 1:8 ) .