John Gill Commentary Proverbs 18:17

"He that pleadeth his cause first [seemeth] just; But his neighbor cometh and searcheth him out." — Proverbs 18:17 (ASV)

He that is first in his own cause [seems] just. As perhaps Tertullus did, before Paul made his defence; and as Ziba, Mephibosheth's servant, before his master detected him: this often appears true in telling a tale, in private conversation, in lawsuits before a judge and a court of judicature, and in theological controversies;

but his neighbour comes, and searches him; his neighbour comes into the house, where he is telling his tale, and reports it in another manner, and shows the falsehood of his relation; or he comes into a court of judicature, and sets the cause in quite another light; or he comes out into the world by public writing, and exposes the errors of a man engaged in a wrong cause, and refutes his arguments.

It is generally understood of judicial affairs, that the first that opens a cause is very apt to prejudice the judge and court in his favour, and they are ready to think at first hearing that he is in the right; but it is not proper to be hasty in forming a judgment till the other side is heard; for his antagonist comes and traverses the point, unravels the whole affair, shows the weakness of his cause, the vanity of his pretences, and makes void all his allegations; and then "he", the judge, so some interpret it, "searches"; inquires more narrowly into the case, in order to find out truth, and pass a right judgment and sentence.