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Responding to a Subpoena
גמ׳ והאמר שמואל דינא דמלכותא דינא

GEMARA: But didn’t Shmuel say: The law of the kingdom is the law, i.e., there is a halakhic principle that Jews must obey the laws of the state in which they live.

(א) וְנֶ֣פֶשׁ כִּֽי־תֶחֱטָ֗א וְשָֽׁמְעָה֙ ק֣וֹל אָלָ֔ה וְה֣וּא עֵ֔ד א֥וֹ רָאָ֖ה א֣וֹ יָדָ֑ע אִם־ל֥וֹא יַגִּ֖יד וְנָשָׂ֥א עֲוֺנֽוֹ׃
(1) If a person incurs guilt— When he has heard a public imprecation and—although able to testify as one who has either seen or learned of the matter—he does not give information, so that he is subject to punishment;

(א) ושמעה קול אלה. בְּדָבָר שֶׁהוּא עֵד בּוֹ — שֶׁהִשְׁבִּיעוּהוּ שְׁבוּעָה, שֶׁאִם יוֹדֵעַ לוֹ בְּעֵדוּת, שֶׁיָּעִיד לוֹ (ספרא):

(1) ‎‏‎ קול האלה ‎ה‎ושמע‎ [AND IF A SOUL SIN] AND HEAR THE VOICE OF AN OATH in a matter to which he was witness, i. e. that he (the person interested in the evidence) called upon him (the witness) by an oath that if he knows any evidence favourable to him he should testify for him before the court (cf. Sifra); if he does not tell it, he bears his iniquity.
(א) ונפש כי תחטא ושמעה קול אלה. הוא החרם וכן ואת אלית והכתוב אחז דרך קצרה והזהיר שחייב העד להגיד כי אם לא יגיד יש עליו עונש מהשם שישא עונו אם שכח ולא הגיד ואחר כן נזכר לו:
(1) anyone who sins, having heard admission of guilt specifically, regarding forbidden articles (as in, “you admitted your guilt” [Judges 17:2]). Although Scripture employs brevity, it implicitly warns the witness that he is obliged to testify, since if he does not testify, he is subject to Divine punishment: he has committed an offense . This applies when the witness forgot to testify, and afterward remembered.
(א) ושמעה קול אלה. בא וראה שבכל הפרשיות כתיב שגגה וכאן ובשבועת בטוי לא כתיב שגגה לומר לך שגדול עונש השבועה שאפילו שגגתה עולה זדון וכן אמר שלמה אל תתן את פיך לחטיא את בשרך ואל תאמר לפני המלאך כי שגגה היא וגם כשיבא הקב"ה לדון את הבריות ידון אותם עם מכשפים ומנאפים שנאמר והייתי עד ממהר במכשפים ובמנאפים ובנשבעים בשמי לשקר:
(1) ושמעה קול אלה, “by having heard an oath demanding him to testify, and having ignored it;” all the other paragraphs distinguish between inadvertently committed sins and those committed deliberately except the one involving an oath, known as שבועת בטוי, “a futile oath,” which is automatically considered as the one who uttered it doing so while aware of its implications. Solomon confirmed this in Kohelet 5,5 when he said: אל תתן את פיך לחטיא את בשרך ואל תאמר לפני המלאך כי שגגה היא, למה יקצוף האלוקים על קולך וחבל את מעשה ידיך, “do not let your mouth bring you in disfavour; and do not plead before the messenger (angel) that it was only an error; but fear the Lord else He may be angered by your talk and destroy your possessions.” When G–d will sit in judgment of all of His creatures, He will place such people next to the sorcerers and adulterers, as we know from the prophet Malachi (Malachi 3,5) והייתי עד ממהר במשפים ובמנאפים ובנשבעים (בשמי) לשקר, “I will act as relentless accuser against those that have no fear of Me, who practice sorcery, commit adultery and who swear falsely (using My name).”
(א) והוא עד או ראה או ידע. אינו ג' עניינים דאי אפשר לו להיות עד בלא ראייה וידיעה אלא שהשביעוהו להעיד בדבר שהוא עד על ידי ראייה בלא ידיעה כגון שטוען מנה לויתי לך בפני פלו' ופלו' ומשביעו שיעיד לו עליו זהו ראייה בלא ידיעה או בדבר שהוא עד בידיעה בלא ראייה כגון שטוען מנה הודית לי בפני פלו' ופלו' ומשביעו שיעיד והרי ידיעה בלא ראייה. א"נ שאמר והוא עד שראה ההלואה ויודע הכל. או ידע פי' שאין כאן אלא ידיעה לבד. ולא אמר כאן ונעלם ממנו כי הוא חייב במזיד או בשגגת שבועה עם זדון העדות אבל אם היה שכוח מן העדות בשעת השבועה פטור:

(1) והוא עד או ראה או ידע, “and he is a witness, either having seen or having definitive knowledge.” The Torah does not speak of three different scenarios here, for it is impossible for someone to be a witness to something that he has neither seen nor otherwise has definitive knowledge of. It must deal therefore with the case of someone beseeching his friend or acquaintance to testify on his behalf to a matter which he has definitive knowledge of by having been an eye witness of it, or having ironclad knowledge of when the matter is not subject to eyesight, such as testifying to words spoken. He may have heard a party admit owing money to another party, for instance. The presence of such a party when money changed hands, would be “seeing” something, the hearing of an admission, would be “knowing” something without the benefit of having seen it. It is also possible to explain the words והוא עד, to mean that this refers to knowledge which includes both having seen what happens and having understood its impact, such as having been present when the loan was handed over and receipt was confirmed by the recipient. The subsequent או ידע would then mean that although the party had been fully aware of the loan having been transacted, the recipient having acknowledged it, he had not seen the money change hands. The Torah, in this case, does not write: ונעלם ממנו, “it was concealed from him,” as it did in the next three verses, seeing that the refusal to testify is not an inadvertent sin but a deliberately committed one. If, at the time when requested to render an oath, the witness had indeed forgotten what he had known at the time, he does not need to render such an oath.

(א) כי תחטא. צריך לדעת לאיזה ענין אמר כי תחטא שלא היה לו לומר אלא כי תשמע קול וגו', ונראה כי להיות שלא הוצרך זה להשביעו לזה אלה לצד שלא רצה להעיד עדות שיש לו לזכותו, כי בתחלה ישאל ממנו עדותו בלא השבעה, ועל כפירתו קורא עליו הכתוב כי תחטא, כי חטא הוא זה לדבר שקרים, ואם שמעה קול וגו' תתחייב בקרבן. ובזה תתיישב וא''ו של ושמעה שיורה שזולת זה הוא החטא הרשום בו חטאה:

(1) כי תחטא, if one sins, etc. Why did the Torah have to introduce this paragraph with the words ונפש כי תחטא ושמעה?, It would have sufficed to write ונפש כי תשמע. Perhaps the reason is that the person who is the subject of this paragraph is one who had previously denied knowing of testimony which could result in an accused's exoneration. When he does so a second time, he proves that he had already incriminated himself previously. The Torah alludes to this state of affairs by writing נפש כי תחטא, someone who has already sinned, etc. The fact that the potential witness had lied already previously is accounted as a sin. All of this is confirmed by the letter ו the beginning of the word ונפש, at the start of this paragraph.

אמר רבא מנלן דמשדרין שליחא דבי דינא ומזמנינן ליה לדינא דכתיב (במדבר טז, יב) וישלח משה לקרא לדתן ולאבירם בני אליאב ומנלן דמזמנינן לדינא דכתיב (במדבר טז, טז) ויאמר משה אל קרח אתה וכל עדתך.

לקמי גברא רבה דכתיב (במדבר טז, טז) לפני ה' את ופלניא דכתיב אתה והם ואהרן דקבעינן זימנא דכתיב מחר זימנא בתר זימנא דכתיב (ירמיהו מו, יז) קראו שם פרעה מלך מצרים שאון העביר המועד.

Having discussed some of the restrictions applying to a person who was ostracized, the Gemara explains some of the basic principles relating to ostracism: Rava said: From where do we derive that a court agent is sent to summon the defendant to appear before the court before he is ostracized? As it is written: “And Moses sent to call Dathan and Abiram, the sons of Eliab” (Numbers 16:12). And from where do we derive that we summon the defendant, that he himself must appear before the court? As it is written: “And Moses said to Korah: Be you and all your congregation before the Lord, you and they, and Aaron, tomorrow” (Numbers 16:16).

From where is it derived that the defendant must be told that he is being summoned to appear before a great man? As it is written: “And Moses said to Korah: Be you and all your congregation before the Lord” (Numbers 16:16). From where is it derived that the summons must mention the names of both parties: You and so-and-so, the plaintiff? As it is written: “You, and they, and Aaron” (Numbers 16:16). From where is it derived that we set a date for the court proceedings? As it is written: “Tomorrow” (Numbers 16:16). From where is it derived that a second date must be set after the first date, if the defendant did not heed the first summons? As it is written: “They did cry there, Pharaoh, king of Egypt is but a noise; he has passed the time appointed…Surely as Tabor is among the mountains, and as Carmel by the sea, so shall he come” (Jeremiah 46:17). Although a time had been appointed and it had passed, it was not canceled entirely but pushed off to a later date.

- Talmud Mo'ed Katan 16a

שליח שאמר פלוני שלחני בשם א' מהדיינים ולא רצה לבוא אין כותבין עליו פתיחא של שמתא עד שיאמר בשם שלשתן ואם יש בהם מומחה חולקין לו כבוד ומזכירין לו שם המומחה (נ"י פ' אלו מגלחין) בד"א שהלך השליח ביום שאינו ידוע לישיבת הדיינים אבל ביום הידוע שהדיינים יושבים בו לדין הכל יודעים שכל הדיינים מקובצים ואע"פ שבא השליח בשם אחד כאלו בא בשם שלשתן:

Choshen Mishpat 11:1

A [Court] messenger who stated, 'So-and-so has sent me [to summon you to Court]' in the name of one of the Judges,19e., So-and-so in whose name the Court messenger summoned the defendant is one of the Judges — M.E. Cf. source of this ruling in n. 23 infra. and [the defendant] refused to come, — [the law is that] they do not write against him a warrant of excommunication20 supra par. 1 and notes. Actually even without writing the warrant of excommunication we do not place the defendant under the ban unless the statements conflict, i.e., if the Court messenger says that he summoned the defendant in the name of the three Judges and the latter denies this in which case we place the defendant under the ban but do not write a warrant (Pethiḥa). , supra § 8, 5, n. 45. until he [the messenger] states [this] in the name of the three [Judges]. And if there is among them an expert [Judge], they accord him honour and one [the Court messenger] mentions to him [only] the name of the expert [Judge].21N.Yos. to M.K. III — G. , M.K. 16a and supra par. 1, n. 1. This has reference even where he summoned the defendant on a day which is not fixed for holding judicial sessions — M.E. and Nethiboth although the matter still requires further elucidation. , Be’er Eliyahu. This ruling, furthermore, applies even if the Judge is not a recognized authority (מומחה לרבים) — Nethiboth. When does this apply? — Where the [Court] messenger went [to summon the defendant] on an [ordinary] day that is not known [to be appointed] for a judicial session, but [if] on the day that is known [to be appointed as a Court-day] when the Judges sit in judgment, everyone knows that all the Judges are assembled [at their meeting place] and although the [Court] messenger came [to summon the defendant] in the name of one [Judge],22Provided he was equal and not inferior to the other two Judges in learning — ShaK on the authority of RaShaL. it is as though he came in the name of the three [Judges].23

How many Hazmanot (summons) are sent? (pg. 182 of "Gray Matter")
The Shulchan Aruch (C.M. 11:1) speaks of sending three hazmanot to a rural resident who occasionally visits the city and one hazmanah to a city dweller. Acharonim (....) note that nowadays we send three hazmanot even to city dwellers, because our lives have become so hectic that we need reminding. In American we follow the practice of always sending three hazmanot. In Israel, the practice has developed to send only one hazmanah, in both the non-government batei din (Badatz) and state-sponsored Israeli Rabbinate batei din.
The practice of the Yemenite batei din is particularly interesting. If a party would respond only to a second or third notice, the dayanim would conduct the proceedings without asking him why he ignored the earlier hazmanot, lest they be biased against him. However, after the final decision was issues, they inquire as to the reason for the delay. If he failed to provide a legitimate excuse for the tardy response, the dayanim would reprimand him.
(ב) לֹֽא־תִהְיֶ֥ה אַחֲרֵֽי־רַבִּ֖ים לְרָעֹ֑ת וְלֹא־תַעֲנֶ֣ה עַל־רִ֗ב לִנְטֹ֛ת אַחֲרֵ֥י רַבִּ֖ים לְהַטֹּֽת׃
(2) You shall neither side with the mighty to do wrong—you shall not give perverse testimony in a dispute so as to pervert it in favor of the mighty

(א) לא תהיה אחרי רבים לרעת. יֵשׁ בְּמִקְרָא זֶה מִדְרְשֵׁי חַכְמֵי יִשְׂרָאֵל, אֲבָל אֵין לְשׁוֹן הַמִּקְרָא מְיֻשָּׁב בָּהֶן עַל אָפְנָיו. מִכָּאן דָּרְשׁוּ שֶׁאֵין מַטִּין לְחוֹבָה בְּהַכְרָעַת דַּיָּן אֶחָד, וְסוֹף הַמִּקְרָא דָּרְשׁוּ אחרי רבים להטת, שֶׁאִם יֵשׁ שְׁנַיִם מְחַיְּבִין יוֹתֵר עַל הַמְזַכִּין הַטֵּה הַדִּין עַל פִּיהֶם לְחוֹבָה – וּבְדִינֵי נְפָשׁוֹת הַכָּתוּב מְדַבֵּר – וְאֶמְצַע הַמִּקְרָא דָּרְשׁוּ לא תענה על רב – עַל רַב, שֶׁאֵין חוֹלְקִין עַל מֻפְלָא שֶׁבְּבֵית דִּין, לְפִיכָךְ מַתְחִילִין בְּדִינֵי נְפָשׁוֹת מִן הַצַּד – לַקְּטַנִּים שֶׁבָּהֶם שׁוֹאֲלִין תְּחִלָּה שֶׁיֹּאמְרוּ אֶת דַּעְתָּם – וּלְפִי דִּבְרֵי רַבּוֹתֵינוּ כָּךְ פִּתְרוֹן הַמִּקְרָא: לא תהיה אחרי רבים לרעת. לְחַיֵּב מִיתָה בִּשְׁבִיל דַּיָּן אֶחָד שֶׁיִּרְבּוּ מְחַיְּבִין עַל הַמְזַכִּין, וְלֹא תַעֲנֶה עַל הָרַב לִנְטוֹת מִדְּבָרָיו – וּלְפִי שֶׁהוּא חָסֵר יוֹ"ד דָּרְשׁוּ בוֹ כֵּן – אחרי רבים להטת, יֵשׁ רַבִּים שֶׁאַתָּה נוֹטֶה אַחֲרֵיהֶם, וְאֵימָתַי? בִּזְמַן שֶׁהֵן שְׁנַיִם הַמַּכְרִיעִין בַּמְחַיְּבִין יוֹתֵר מִן הַמְזַכִּין; וּמִמַּשְׁמָע שֶׁנֶּאֱמַר לא תהיה אחרי רבים לרעת שׁוֹמֵעַ אֲנִי אֲבָל הֱיֵה עִמָּהֶם לְטוֹבָה, מִכָּאן אָמְרוּ דִּינֵי נְפָשׁוֹת מַטִּין עַל פִּי אֶחָד לִזְכוּת וְעַל פִּי שְׁנַיִם לְחוֹבָה. וְאֻנְקְלוֹס תִּרְגֵּם לָא תִתְמְנַע מִלְּאַלָּפָא מָה דְמִתְבְּעֵי לָךְ (דִבְעֵינָךְ) עַל דִּינָא, וּלְשׁוֹן הָעִבְרִי לְפִי הַתַּרְגּוּם כָּךְ הוּא נִדְרָשׁ: לא תענה על רב לנטת. אִם יִשְׁאָלוּךָ דָּבָר לַמִּשְׁפָּט לֹא תַעֲנֶה לִנְטוֹת לְצַד אֶחָד וּלְסַלֵּק עַצְמְךָ מִן הָרִיב, אֶלָּא הֱוֵי דָן אוֹתוֹ לַאֲמִתּוֹ. (ב) וַאֲנִי אוֹמֵר לְיַשְּׁבוֹ עַל אָפְנָיו כִּפְשׁוּטוֹ כָךְ פִּתְרוֹנוֹ: לא תהיה אחרי רבים לרעת. אִם רָאִיתָ רְשָׁעִים מַטִּין מִשְׁפָּט, לֹא תֹאמַר, הוֹאִיל וְרַבִּים הֵם הִנְנִי נוֹטֶה אַחֲרֵיהֶם: (ג) ולא תענה על רב לנטת וגו'. וְאִם יִשְׁאָלְךָ הַנִּדּוֹן עַל אוֹתוֹ הַמִּשְׁפָּט, אַל תַּעֲנֶנּוּ עַל הָרִיב דָּבָר הַנּוֹטֶה אַחֲרֵי אוֹתָן רַבִּים לְהַטּוֹת אֶת הַמִּשְׁפָּט מֵאֲמִתּוֹ, אֶלָּא אֱמֹר אֶת הַמִּשְׁפָּט כַּאֲשֶׁר הוּא וְקוֹלָר יְהֵא תָלוּי בְּצַוַּאר הָרַבִּים (סנהדרין ז'):

(1) לא תהיה אחרי רבים לרעת THOU SHALT NOT FOLLOW THE MANY FOR EVIL — There are Halachic interpretations of this verse given by the Sages of Israel but the wording of the text does not fit in well with them. They derive from here that we must not decide a person’s guilt by a preponderance of one judge. And the end of the verse they explained thus: אחרי רבים להטות — but if the judges who declare the defendant guilty are two more than those who declare him innocent, then decide the matter as they declare — that he is guilty (Sanhedrin 2a). — The verse, they point out, speaks of capital cases. — The middle passage לא תענה על רב, they explained as though it were written על רַב, “thou shalt not speak against the chief of the judges, meaning that one should not give an opinion different from that given by the מופלא of the court (the most eminent among the judges, because this is disrespectful to the Presiding-judge). In consequence of this rule we begin to take the view of those in the side-benches first — we ask the youngest judges to express their opinion first (so that they may not be able to vote against the view expressed by the מופלא). Therefore the exegesis of the verse according to the words of our Rabbis is as follows: “thou shalt not follow a bare majority for evil” — to sentence a man to death on account of the one judge by whom those who condemn him are more in number than those who acquit him; “and thou shalt not speak against the chief inclining away” from his opinion. — They explained this latter phrase thus, because the word which is usually written רִיב is here written without and therefore may be read, אחרי רבים להטות — ;רַב, there is, however, a majority to whose view thou must incline. When is this the case? When there are two who preponderate amongst those who vote for condemnation over and above those who vote for acquittal. For from what is implied in, “thou shalt not follow a bare majority for evil”, I may infer: but thou shall follow it for good. Hence they (the Rabbis) said (i. e. they established the general rule): In capital cases we may decide by a majority of one for acquittal, but only by a majority of at least two to condemn. Onkelos translates the second phrase by: Do not refrain from teaching when you are being asked your opinion in a legal matter. The Hebrew text is to be explained according to the Targum as follows: לא תענה על רב לנטת If you are being asked your opinion in a legal matter do not give your answer just to incline to one particular side and so to withdraw yourself from the dispute, but decide the matter as truth requires. Such are the expositions that have been offered of this verse. (2) But I think that if one wishes to explain the verse so that every thing should fit in properly, its exegesis must be as follows: לא תהיה אחרי רבים לרעת, If you see wicked men wresting judgment do not say: since they are many I will incline after them; (3) ולא תענה על רב לנטת וגו׳, and if the defendant asks you about that judgment do not give him as a reply concerning the dispute any statement which will incline after that majority, thereby wresting judgment from the truth, but pronounce the decision just as it should be and let the collar hang around the neck of the majority (i. e. if you be outvoted let them bear the responsibility).

(1) לא תהיה אחרי רבים לרעות, as the tie-breaking vote in a trial involving capital punishment. One cannot declare someone guilty of the death penalty on the basis of a solitary judge. A majority of one would be equivalent to a conviction by a single judge. (2) ולא תענה על ריב, when your colleagues the other judges ask your opinion, (3) לנטות אחרי רבים, do not be influenced by the fact that the majority thinks differently from you. Assuming that in a trial 10 of the 23 judges had expressed the view that the accused was innocent whereas eleven had expressed the view that the accused was guilty, (4) להטות, if you were to cast your vote with the majority there would then be a majority of 2 out of a total of 23 voting guilty, and that decision would have been arrived at by your single vote. You are not allowed to salvage your conscience by voting with the majority unless this represented your absolute conviction. You must explain the reason for your vote. Unless there is a majority of two votes in favour of guilty no one can be convicted of the death penalty.

(א) ולא תענה על רב אפילו אם אתה חריף בעיניך להראות טעם לנטות הדין וחבריך אינם חריפין כמוך לירד לעומק הדין הכתוב מזהירך שלא תענה אותו טעם לנטותם. (ב) אחרי רבים להטת אלא צריך שיגמר הדין אחר הרוב.
(1) ולא תענה על ריב, “and do not ignore the majority view seeing you consider yourself as smarter than your colleagues, when this would result in perverting justice;” (2) אחרי רבים להטות, “but make certain that the verdict is based on a majority of the judges’ opinions.”
"To sum up the matter, the principle that no man is to be declared guilty on his own admission is a divine decree." Maimonides, Mishneh Torah (Code of Jewish Law), Book of Judges, Laws of the Sanhedrin
Encyclopedia Judaica: Confession
Along with admissions of fact from which any criminal responsibility may be inferred, confessions are not admissible as evidence in criminal or quasi-criminal proceedings, for "no man may call himself a wrongdoer" (Sanh. 9b). This rule against self-incrimination developed from the rule that a wrongdoer is incompetent as a *witness , being presumed to be unjust and untruthful (cf. Ex. 23:1). Since some people might admit to misconduct in order to disqualify themselves from testifying, to cure this mischief the rule was laid down that no man can be heard to say of himself that he is so guilty as to be an incompetent witness (Sanh. 25a; BK 72b). The rule was originally derived from the principle that no man is competent to testify in his own favor (Ket. 27a) – his confession being intended to confer the benefit of not being required to testify.
The rule against self-incrimination dates only from talmudic times. Several instances of confessions are recorded in the Bible (e.g., Josh. 7:19–20; II Sam. 1:16; cf. I Sam. 14:43), but these are dismissed by talmudic scholars either as confessions after trial and conviction, made for the sole purpose of expiating the sin before God (Sanh. 43b), or as exceptions to the general rule (hora'at Sha'ah; cf. Maim. comm. to the Mishnah, Sanh. 6:2; Ralbag to II Sam. 1:14). As all instances recorded in the Bible related to proceedings before kings or rulers, it may be that they did not consider themselves bound to observe regular court procedures (cf. Maim. Yad, Melakhim 3:10). Confessions are inadmissible not only in capital cases, but also in cases involving only *flogging , *fines (Rashi to Yev. 25b), or quasi-punishments (ibid.; cf. Resp.Rosh 11:5). Opinions are divided on whether a *ḥerem and public admonitions could be administered on the strength of a confession only.
Varying reasons were given for the rule against self-incrimination: the earliest and commonest is that the biblical requirement of the evidence of at least two witnesses for the condemnation of any man (Deut. 17:6; 19:15) implicitly excludes any other mode of proof (Tosef., Sanh. 11:1, 5). Maimonides adds that melancholy and depressed persons must be prevented from confessing to crimes which they have not committed so as to be put to death (Yad, Sanhedrin 18:6). Another theory was based on the prophet's words that all souls are God's (Ezek. 18:4), hence no man may be allowed to forfeit his life (as distinguished from his property) by his own admission, his life not being his own to dispose of but God's (David b. Solomon ibn Abi Zimra); still another scholar held that if confessions were accorded any probative value at all, courts might be inclined to overrate them, as King David did (II Sam. 1:16), and be guilty of a dereliction of their own fact-finding task (Joseph ibn Migash). A 19th-century jurist (Mordechai Epstein) pointed out that the real difference between civil admissions and criminal confessions was that by an admission an obligation was created which had only to be enforced by the court, whereas in a criminal conviction it is the court which creates the accused's liability to punishment. While it is nowhere expressed, the reason for the exclusion of confessions may well have been the desire to prevent their being elicited by torture or other violent means: it is a fact that – unlike most contemporaneous law books – neither Bible nor Talmud provide for any interrogation of the accused as part of the criminal trial, so that there was no room for attempts to extort confessions.
[Haim Hermann Cohn]

Mishnah Yomit 4:8

If a man declares, “this son of mine is a mamzer,” he is not believed. There are two explanations given for why a father is not believed to say that his child is a mamzer. The first is that the father is related to the child and relatives may not testify. The second is that by saying that the child is a mamzer, the father is testifying against himself and halakhah does not allow self-incrimination.

Introductions to the Babylonian Talmudהקדמות לתלמוד הבבלי
Ketubot, Summary of Perek II
There are two restrictions to this form of credibility. First, it is accorded only when there is no independent source of information with regard to that matter. When there is such information, i.e., witnesses or some other source of objective information that is incriminating, the fact that the party admitted it but then negated its significance does not grant him credibility. Second, when a party's claim involves self-incrimination or is suspect of being false for some external reason, it is not granted credibility.