Objection 1. It would seem unlawful for a judge to pronounce judgment against the truth that he knows, on account of evidence to the contrary. For it is written (Deuteronomy 17:9): "Thou shalt come to the priests of the Levitical race, and to the judge that shall be at that time; and thou shalt ask of them, and they shall show thee the truth of the judgment." Now sometimes certain things are alleged against the truth, as when something is proved by means of false witnesses. Therefore it is unlawful for a judge to pronounce judgment according to what is alleged and proved in opposition to the truth which he knows.
Objection 2. Further, in pronouncing judgment a man should conform to the Divine judgment, since "it is the judgment of God" (Deuteronomy 1:17). Now "the judgment of God is according to the truth" (Romans 2:2), and it was foretold of Christ (Isaiah 11:3-4): "He shall not judge according to the sight of the eyes, nor reprove according to the hearing of the ears. But He shall judge the poor with justice, and shall reprove with equity for the meek of the earth." Therefore the judge ought not to pronounce judgment according to the evidence before him if it be contrary to what he knows himself.
Objection 3. Further, the reason why evidence is required in a court of law, is that the judge may have a faithful record of the truth of the matter, wherefore in matters of common knowledge there is no need of judicial procedure, according to 1 Timothy 5:24, "Some men's sins are manifest, going before to judgment." Consequently, if the judge by his personal knowledge is aware of the truth, he should pay no heed to the evidence, but should pronounce sentence according to the truth which he knows.
Objection 4. Further, the word "conscience" denotes application of knowledge to a matter of action as stated in I, 79, 13. Now it is a sin to act contrary to one's knowledge. Therefore a judge sins if he pronounces sentence according to the evidence but against his conscience of the truth.
On the contrary, Augustine [Ambrose, Super Ps. 118, serm. 20 says in his commentary on the Psalter: "A good judge does nothing according to his private opinion but pronounces sentence according to the law and the right." Now this is to pronounce judgment according to what is alleged and proved in court. Therefore a judge ought to pronounce judgment in accordance with these things, and not according to his private opinion.
I answer that, As stated above (1; 60, 2,6) it is the duty of a judge to pronounce judgment in as much as he exercises public authority, wherefore his judgment should be based on information acquired by him, not from his knowledge as a private individual, but from what he knows as a public person. Now the latter knowledge comes to him both in general and in particular --in general through the public laws, whether Divine or human, and he should admit no evidence that conflicts therewith--in some particular matter, through documents and witnesses, and other legal means of information, which in pronouncing his sentence, he ought to follow rather than the information he has acquired as a private individual. And yet this same information may be of use to him, so that he can more rigorously sift the evidence brought forward, and discover its weak points. If, however, he is unable to reject that evidence juridically, he must, as stated above, follow it in pronouncing sentence.
Reply to Objection 1. The reason why, in the passage quoted, it is stated that the judges should first of all be asked their reasons, is to make it clear that the judges ought to judge the truth in accordance with the evidence.
Reply to Objection 2. To judge belongs to God in virtue of His own power: wherefore His judgment is based on the truth which He Himself knows, and not on knowledge imparted by others: the same is to be said of Christ, Who is true God and true man: whereas other judges do not judge in virtue of their own power, so that there is no comparison.
Reply to Objection 3. The Apostle refers to the case where something is well known not to the judge alone, but both to him and to others, so that the guilty party can by no means deny his guilt (as in the case of notorious criminals), and is convicted at once from the evidence of the fact. If, on the other hand, it be well known to the judge, but not to others, or to others, but not to the judge, then it is necessary for the judge to sift the evidence.
Reply to Objection 4. In matters touching his own person, a man must form his conscience from his own knowledge, but in matters concerning the public authority, he must form his conscience in accordance with the knowledge attainable in the public judicial procedure.
Source: Summa Theologica (New Advent)