Rambam - 1 Chapter a Day

Sanhedrin 27b.

Sefer HaMitzvot (negative commandment 287) and Sefer HaChinuch (mitzvah 589) count this as one of the 613 mitzvot of the Torah. As the Rambam states at the conclusion of the chapter, we do not disqualify relatives because we fear that they will lie on behalf of their relative. Instead, this is a Scriptural decree; Moses would not be able to testify on behalf of Jethro, his father-in-law (Bava Batra 159a).

Just as two relatives may not testify on behalf of each other; so, too, they may not testify together on behalf of someone else. Although the verse speaks about testimony in cases involving capital punishment, it applies with regard to all types of testimony.

As the Rambam proceeds to explain, if Jacob fathered Reuven and Shimon, Reuven fathered David, and Shimon fathered Pinchas, none of the individuals mentioned can testify on behalf of each other.

See Halachah 5.

I.e., Pinchas and David may not testify together.

I.e., Reuven may not testify with Pinchas. Diagram

The Rambam’s statements with regard to maternal relatives have attracted the attention the commentaries, for Sanhedrin 28a derives the disqualification of maternal relatives from the exegesis of a verse, implying that the prohibition is of Scriptural origin. This is in fact the position of many other Rishonim.
The Radbaz (- and this interpretation also appears to fit the Rambam’s Commentary to the Mishnah -) explains that although this perspective is mentioned by the Talmud, it is not accepted as halachah. The Sefer Me’irat Einayim 33:7 states that the Rambam interprets that Talmudic passage as an asmachta, i.e., a support found by the Sages for a law that was part of the Oral Tradition.
The Kessef Mishneh explains that the Rambam maintains - as he writes in his Sefer HaMitzvot (General Principle 2) - that laws derived through the principles of Biblical exegesis are midivrei sofrim, “from the words of the Sages.” Nevertheless, such laws have the same authority as if they were stated explicitly in the Torah. See the Ramah (Choshen Mishpat 33:2) and commentaries where this issue is discussed.

Who came from the same drop of sperm (Yevamot 98a).

The Kessef Mishneh notes that the phrase “that they convert” implies that they converted themselves. If, however, a women converts while pregnant with twin sons, since they were born as Jews, they are disqualified from testifying on each other’s behalf by Rabbinic decree. The Siftei Cohen 33:7 cites the Rambam’s ruling in Hilchot Issurei Bi’ah 14:14 which states that, in such a situation, the brothers are forbidden to have relations with each other’s wife by Scriptural Law. Accordingly, he maintains that they are also disqualified as witnesses by Scriptural Law. The Be’er Heitev 33:7, however, explains that a distinction can be made between the two instances.

Without any connection at all to his past. Indeed, according to Scriptural Law, a convert can marry his mother or his sister (Hilchot lssurei Bi’ah 14:12).
Our Sages did not decree that converts may not testify on behalf of their relatives, lest one permit native-born Jewish relatives to testify on each other’s behalf, because the acceptance of witnesses is dependent on the court and a court would not make such an error (Yevamot 22a).

To refer back to the situations mentioned in Halachah 1. Jacob fathered Reuven and Shimon, Reuven fathered David, Shimon fathered Pinchas, David fathered Solomon, and Pinchas fathered Eli. Eli may testify on behalf of Reuven. Diagram

Eli may testify on behalf of Solomon.

David and Pinchas.

David and Shimon.

Sanhedrin 27b derives this concept from the exegesis of the verse: “Fathers shall not die because of sons.” Rashi explains that since the Torah uses the plural term for both father’s and sons, the implication is that several sets of relatives are involved.

For he is considered as being two degrees removed.

Rabbenu Tam states that a person may not testify together with his great-grandson. The Shulchan Aruch (Choshen Mishpat 33:2) quotes both opinions and the Ramah states that Rabbenu Tam’s view should be followed.

And needless to say, the sons of their daughters and the daughters of their sons.

Sanhedrin 28b derives this concept from the exegesis of Leviticus 18:14.

The opinions which disqualify a person from testifying on behalf of his great-grandson grant leniency and allow him to testify on behalf of his great-grandson’s wife, provided there is no direct monetary benefit to his great-grandson involved [Shulchan Aruch and Ramah (Choshen Mishpat 33:3)].

To cite an example: Jacob gave birth to Dinah and Devorah, Dinah gave birth to Sarah, and Devorah, to Leah. Sarah married Abraham and Leah married Joel, Abraham and Joel may testify on behalf of each other. Diagram

When relatives are distantly removed, we do not employ the concept: "A husband is considered like his wife" twice in the same instance to disqualify a witness (Radbaz).
The Ramah (Choshen Mishpat 33:4) quotes the Terumat HaDeshen who rules that the initial and preferred option is for individuals who are related by marriage in this manner not to be appointed as witnesses together or to sign a legal document together as witnesses.

To continue using the above example: Dinah married Joseph. Joseph and Abraham may not testify on behalf of each other.

Since the two women are not distantly removed, we employ the concept: “A husband is considered like his wife” twice in the same instance to disqualify a witness.

To refer to the above example: Devorah married Baruch. Joseph and Baruch may not testify on each other’s behalf.

To refer to the above example, if Devorah had a son, Michael, Joseph could not testify on behalf of Michael. In this instance, after using the principle: “A husband is considered like his wife,” Joseph is considered as one degree removed and Michael, two degrees removed. Hence, they are disqualified to testify on behalf of each other.

To refer to the above example, if Devorah had a daughter, Judith who married Judah, Joseph could not testify on behalf of Judah. In this instance, after using the principle twice: “A husband is considered like his wife,” Joseph is considered as one degree removed and Judah, two degrees removed. Hence, they are disqualified to testify on behalf of each other.
The Shulchan Aruch (Choshen Mishpat 33:4) quotes the Rambam’s ruling on this point, but also quotes the opinion of Rabbenu Asher who allows such testimony. The Ramah rules that Rabbenu Asher’s opinion should be followed.

To refer to the above example: Baruch had fathered Aaron in a previous marriage. Joseph may testify on Aaron’s behalf.

I.e., if Eliezer and Rebbeca are brother and sister and Rebbeca is married to Isaac, Eliezer may not testify on behalf of Isaac. He may, however, testify on behalf of all of Isaac’s relatives. For they share no family connection at all. See Halachah 12.

I.e., if Eliezer’s son, Dan, marries Osnat, Eliezer may testify on behalf of all of Osnat’s relatives.

For the two individuals themselves do not share any family connection. Sanhedrin 28b describes them as “lids to jugs.”
In their glosses to the Shulchan Aruch (Choshen Mishpat 33:6), Rabbi Akiva Eiger and the Chatam Sofer rule that this law applies even after nissuin (the consummation of the marriage bond). See also the Ramah (Choshen Mishpat, Ibid.) who states that as an initial and preferred option, these individuals should not serve as judges together.

This law, based on Sanhedrin 28b, is the source for the principle stated in Halachah 10 (Kessef Mishneh).

For all these principles are one degree removed from his wife and thus two degrees removed from him. And with regard to their husbands or wives, we apply the principle: “A husband is considered like his wife and a wife is considered like her husband.”
The Sefer Me’irat Einayim 33:15-16 emphasizes that with this halachah, the Rambam is clarifying that although we say “a husband is considered like his wife,” they are not considered as the same person. For were that to be true, a husband would also not be able to testify on behalf of her grandson or grandfather according to the Rambam’s perspective.

For we do not assume that he has already become close to them (Sanhedrin 28b). The Rambam’s wording implies that the initial and preferred option is not to have such a person serve as a witness. If he does, however, his testimony is not disqualified. Rabbeinu Asher is more lenient and allows the husband-to-be to serve as a witness for these people at the outset (Sefer Me’irat Einayim 33:17).
The Ramah (Choshen Mishpat 33:9) emphasizes that these laws apply with regard to a woman who was consecrated. If, however, a couple were merely engaged, the husband-to-be is not automatically disqualified. With regard to financial matters, however, it is likely that he would not be allowed to testify because he would be considered an involved party.

As stated above, Moses would not be able to testify on behalf of Jethro, his father-in-law.

For we do not suspect that they will violate the Torah’s prohibitions against testifying falsely because of their love or hatred for the other person.

As stated in the notes to Hilchot Sanhedrin 23:6, we fear that he will agree with the opinion of the person he loves and differ with that of the person he hates. Even though he will not consciously decide to deviate from the path of justice, his subconscious may motivate him to do so.