Rambam - 1 Chapter a Day

Sefer HaMitzvot (negative commandment 288) and Sefer HaChinuch (mitzvah 523) count this as one of the 613 mitzvot of the Torah.

See Sifri to the above verse; Sh’vuot 40a.

If the plaintiff’s claim is supported by the testimony of one witness, the defendant is required to take an oath if he denies the claim.

Chapter 1, Halachah 1.

A woman suspected of adultery. See Numbers, ch. 5; Hilchot Sotah.

I.e., if one witness testifies that he observed her infidelity, she is forced to accept a divorce. She may not drink the waters to prove her innocence. The commentaries explain the rationale for this exception as follows: Since a warning was issued and disobeyed, there is a great likelihood that she was indeed guilty of infidelity. Therefore all testimony is acceptable.

To atone for a murder when the identity of the murderer is unknown. See Deuteronomy, ch. 21; Hilchot Rotzeach, ch. 9.

As Hilchot Rotzeach 9:12 states, if one witness says that he saw the murder, we do not bring this unique atonement offering. For that offering was ordained only when no one at all knew the identity of the murderer, as indicated by Deuteronomy 21:1 (Radbaz).

A woman may not remarry until it is established that her previous husband died. The testimony of one witness is acceptable regarding this matter. As the Rambam states in Hilchot Gerushin 12:15: “If one witness comes and testifies that a woman’s husband died, she is granted permission to marry by virtue of his testimony, because [the truth of] the matter will ultimately be revealed,” i.e., if the husband is alive, that fact will surface in the near future. Hence there is no fundamental need for testimony.
The Rambam’s statements here, however, appear to be slightly contradictory to his statements at the conclusion of Hilchot Gerushin (13:5):
Do not wonder at the fact that our Sages released the prohibition [against marriage to a married woman]... on the basis of the testimony of a woman, a servant or a maidservant.... [These leniencies were instituted] because the Torah requires the testimony of two witnesses... only with regard to matters that cannot be verified definitively except via witnesses and their testimony, e.g., one person killed another or one person lent money to another.
When, by contrast, the matter may be verified definitively without the testimony of a witness and the witness cannot justify [his statements] if they are not true, e.g., in an instance such as this when one testifies that a person died, the Torah did not require [formal testimony].
For this reason, our Sages ruled leniently with regard to this matter and accepted the testimony of one witness that is based on the testimony of a maidservant, [testimony] from a written document, and [testimony] that was not investigated by the ordinary process of interrogation. [These leniencies were instituted] so that the daughters of Israel will not be forced to remain unmarried.
The Noda BiY’hudah (Even HaEzer, Vol. 1, Responsa 27 and 33) interprets the Rambam’s statements as meaning that according to Scriptural Law, the testimony of one witness is sufficient (as in Hilchot Kiddush HaChodesh 3:14 and Hilchot Yibbum VaChalitzah 4:31). Our Sages extended that leniency and also accepted the testimony of one witness based on the testimony of another witness and even one witness based on the testimony of a maid-servant. For as the Rambam states, in these instances, formal testimony is not necessary. His interpretation is, however, difficult to accept in the light of the Rambam’s statements here.

See Chapters 9-16 which describe the reasons why a person may be disqualified from testifying as a witness.

When the testimony of one witness is effective, the requirements of formal testimony are not enforced. Hence, even persons who are otherwise disqualified may offer that information. There are, however, restrictions. A person who is known to be a robber is not accepted as a witness even in these circumstances (Hilchot Gerushin 12:17).

For the obligation to take an oath mandated by the testimony of one witness is comparable to the financial obligation created by the testimony of two witnesses. Hence all the requirements of formal testimony are required.

In some manuscript copies and early printings of the Mishneh Torah, this marks the beginning of Halachah 4.

I.e., related to the other witnesses or to one of the involved parties.

For the witnesses are considered as a single group.

I.e., certainly the fact that the brothers were both present should not prevent justice from being done and the offender from being prosecuted.

The Sefer Me’irat Einayim 36:2 states that according to the Rambam [and the Shulchan Aruch (Choshen Mishpat 36:1)], this question is addressed to all the witnesses who come to testify, not only the relatives. In this way, the unacceptable witnesses will not feel that they are under suspicion and will answer genuinely. Rashi and the Tur (Choshen Mishpat 36) differ and maintain that this question is asked only of the unacceptable witnesses.

Rabbenu Asher and the Tur (Choshen Mishpat 36) follow a slightly different perspective and maintain that relatives are not disqualified unless they both observed the matter and came to court with the intent of testifying. If, however, they merely observed the matter with the intent of testifying, that is not sufficient to disqualify the testimony.
The Shulchan Aruch (Choshen Mishpat 36:1) states that if acceptable witnesses are set aside for that purpose by the involved parties, they are not disqualified even if there are non-acceptable witnesses who observed the event with the intent to testify.
According to all views, there is a difficulty, for seemingly, the relatives of a killer could protect him by coming to court with the acceptable witnesses and saying they observed the matter with the intent of serving as witnesses. In this way, they would disqualify the acceptable witnesses and enable the killer to be released.

I.e., even if the non-acceptable witnesses did not actually testify. Since they intended to testify, all of the witnesses are considered part of a single group. Hence they are all disqualified (Sefer Me’irat Einayim 36:4).
The Ramah (Choshen Mishpat 36:1) states that this restriction applies even when the witnesses do not know of each other’s existence. The Sefer Me’irat Einayim 36:5 questions if the Rambam would also accept this ruling.

See Hilchot Sanhedrin 12:2.

Thus according to the Rambam [and his opinion is accepted by. the Shulchan Aruch (loc. cit.)], it is not necessary for a person who observes an event to watch with the intent of serving as a witness. Even if he decides to serve as a witness afterwards, as long as he can describe the particulars precisely, his testimony is acceptable.

Of either of the principles.

For as is evident from the continuation of the Rambam's statements, it is possible that the unacceptable witnesses signed merely to fill up the space on the legal document without intending to serve as witnesses. Hence if the witnesses say that they all signed together, the legal document is nullified. If they say that the unacceptable witness signed afterwards, the document is acceptable.
The Kessef Mishneh questions how the statements of the witnesses can be used to nullify the legal document. Seemingly, this contradicts the decision rendered previously in Chapter 3, Halachah 7, that once the validity of the signatures on a legal document have been verified, the witnesses cannot nullify the document through their testimony. The Kessef Mishneh explains that in this instance, the statements of the witnesses are accepted, because a question was already raised concerning the validity of the legal document.

Because the unacceptable witnesses signed with the intent of testifying.

After all, non-acceptable witnesses signed the document.

Hence the later signatures are not considered part of the original testimony and do not disqualify it. If all of the witnesses sit down together and sign, their signatures are considered as part of one testimony. If, by contrast, signatures were added afterwards, they do not have bearing on the status of the original signatures. See also the Kessef Mishneh who compares the Rambam’s ruling here to his ruling in Hilchot Gerushin 9:27.

For in actual fact, it is possible that another witness signed before him and the unacceptable witness signed merely to fill in the space.
The Ramah (Choshen Mishpat 45:12) states that at the outset, relatives and unacceptable witnesses should never sign a legal document. Moreover, he also cites the opinion of the Tur who states that the document is not acceptable unless the signatures of the last two witnesses who appear are those of acceptable witnesses. Note the Siftei Cohen 45:23 who elaborates in disputing the conception that the last signatures are those of primary importance.

Sefer HaMitzvot (negative commandment 291) and Sefer HaChinuch (mitzvah 411) count this as one of the 613 mitzvot of the Torah.

Rashi (Sanhedrin 34a) explains the rationale for this prohibition. It appears that he is trying either to support or to retract his original testimony with these statements. This is unnecessary and undesirable. Once a witness delivers testimony, he can no longer retract it or add to it.

The rationale is that all testimony must be able to be disqualified through hazomah. If the witness serves as the judge, he will never disqualify his own testimony [Tosafot (Bava Kama 90b); see also Sefer HaMitzvot, loc. cit.]. The Rashbam (Bava Batra 114a) derives this concept from Deuteronomy 19:17: “And the two men will stand... before God.” “Before God” refers to the judges. The two witnesses must stand before the judges, they cannot become judges themselves.
As evident from Hilchot Sanhedrin 3:6 and Hilchot Kiddush HaChodesh 6:2, if there is no necessity to deliver testimony, the judges can execute the matter themselves, e.g., judges who saw the new moon can consecrate the coming month.

As the Shulchan Aruch (Choshen Mishpat 7:5) emphasizes, this applies when the person testifies as a witness. If, however, a person observes a matter - even if he had the intent to serve as a witness - but does not actually testify, he may serve as a judge.

As Hilchot Gerushin 7:5 states, a person who brings a bill of divorce from the diaspora is required to make this statement before presenting the woman with the document.

And from this time onward, the husband cannot protest that the bill of divorce was a forgery. Since the necessity to validate the signature of witnesses on a legal document is a Rabbinic requirement, our Sages did not prevent a witness from serving as a judge in such an instance.

See also Chapter 7, Halachah 6.