A protest against another fault in judging. Haste is hardly less evil than corruption. "Audi alteram partern "should be the rule of every judge.
His neighbor - The other party to the suit "searcheth," i. e., scrutinizes and detects him.

He that is first in his own cause - Any man may, in the first instance, make out a fair tale, because he has the choice of circumstances and arguments. But when the neighbor cometh and searcheth him, he examines all, dissects all, swears and cross-questions every witness, and brings out truth and fact.

(k) [He that is] first in his own cause [seemeth] just; but his neighbour cometh and searcheth him.
(k) He who speaks first, is best heard from the wicked judge, but when his adversary enquires out the matter it turns to his shame.

He that is first in his own cause seemeth 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 cometh, and searcheth 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 worm 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 thing 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, "searcheth"; inquires more narrowly into the case, in order to find out truth, and pass a right judgment and sentence.

It is well to listen to our enemies, that we may form a better judgment of ourselves.

One-sided statements are not reliable.
searcheth--thoroughly (Proverbs 17:9, Proverbs 17:19).

17 He that is first in his controversy is right;
But there cometh another and searcheth him thoroughly -
an exhortation to be cautious in a lawsuit, and not to justify without more ado him who first brings forward his cause, and supports it by reasons, since, if the second party afterwards search into the reasons of the first, they show themselves untenable. הראשׁון בּריבו are to be taken together; the words are equivalent to אשׁר יבא בריבו בראשׁונה: qui prior cum causa sua venit, i.e., eam ad judicem defert (Fl.). הראשׁון may, however, also of itself alone be qui prior venit; and בריבו will be taken with צדיק: justus qui prior venit in causa sua (esse videtur). The accentuation rightly leaves the relation undecided. Instead of יבא (יבא) the Kerı̂ has וּבא, as it elsewhere, at one time, changes the fut. into the perf. with ו (e.g., Proverbs 20:4; Jeremiah 6:21); and, at another time, the perf. with ו into the fut. (e.g., Psalm 10:10; Isaiah 5:29). But here, where the perf. consec. is not so admissible, as Proverbs 6:11; Proverbs 20:4, the fut. ought to remain unchanged. רעהוּ is the other part, synon. with בעל דין חברו, Sanhedrin 7b, where the אזהרה לבית־דין (admonition for the court of justice) is derived from Deuteronomy 1:16, to hear the accused at the same time with the accuser, that nothing of the latter may be adopted beforehand. This proverb is just such an audiatur et altera pars. The status controversiae is only brought fairly into the light by the hearing of the altera pars: then comes the other and examines him (the first) to the very bottom. חקר, elsewhere with the accus. of the thing, e.g., ריב ,., thoroughly to search into a strife, Job 29:16, is here, as at Job 28:11, connected with the accus. of the person: to examine or lay bare any one thoroughly; here, so that the misrepresentations of the state of the matter might come out to view along with the reasons assigned by the accuser.