In Judaism, the presumption of priestly descent is the presumption that a Jewish man is a priest (kohen), based not on genealogical records of descent from Aaron or on Jewish court rulings, but rather by observation of his priestly behavior as recognized by his peers and community. Such an individual is called a kohen muhzak (presumed kohen or status-quo kohen; Hebrew: כהן מוחזק, from חזק).

The criteria for this determination are described in rabbinical halakhic texts. In the Land of Israel the criteria consisted of performing the Priestly Blessing and receiving terumah at the threshing floor, while in Syria and Babylonia the Priestly Blessing constituted adequate grounds without receiving terumah.^[1][2] Other criteria might include observing the priestly laws about impurity and forbidden marriages, or receiving the first aliyah in synagogue.^[3]

Rabbi Jose ben Halafta extolled the soundness of this "presumption" (chazakah) by calling it a basis for the entire halakhic concept of chazakah.^[4] It is based on this presumption that all poskim agree to forbid presumptive kohanim from marrying a divorcee. Among the Acharonim, this presumption described as "a sound presumption".^[5]

The later books of the Bible describe the use of lineage documents to prove priestly descent,^[6] along with other recordings of lineage.^[7]

The Talmud gives little information regarding the content and form of the lineage document, in contrast to other Rabbinic documents that are described in greater length (for example the Ketubah, Get, business documents (Shtarei Kinyan), and the document of freedom for a bondsman (Shtar Shichrur)).

Rashi mentions that when the Israelites were required to ascertain their lineage to join their respective tribe, this document was brought in tandem with the testimony of witnesses.^[8]

Yair Bacharach noted that the lineage document was not commonly used by kohanim. He argues that the testimony of two Kosher witnesses regarding a kohen's lineage makes him eligible for service.^[9] Similarly, the Tur states that two witnesses are sufficient to qualify a kohen for service on the Mizbeach.^[10] According to the Avodah Tamma, either a lineage document or the testimony of two witnesses was sufficient.

The Mishnah states that during the Temple era, the Sanhedrin would rule on priestly status,^[11] while the Sifri says that the priestly court was responsible.^[12]

With the destruction of the Second Temple and the exile, hard-copy lineage recording was lost. In its place, kohanic legitimacy was determined based on the presumption, as well as the absence of a disqualifying objection by two witnesses in Beit Din.

Upon the return of Jews from the Babylonian exile, and the rebuilding of the Temple in Jerusalem, it became necessary to determine which individuals were actually valid kohanim (descended from Aaron in a direct paternal line, and not descended from the marriage of a kohen to a divorcee or similar forbidden marriages). Thus, Nehemiah conducted the first recorded investigation into the authenticity of apparently legitimate kohanim:

Of the priests: the sons of Hobaiah, the sons of Hakkoz, the sons of Barzillai, who took a wife of the daughters of Barzillai, the Gileadite, and was named after them. These searched among their ancestral registration, but it could not be located; therefore they were considered unclean and excluded from the priesthood. The governor said to them that they should not eat from the most holy things until a priest arose with Urim and Thummim.^[13]

At first glance, it would appear from these verses that the mentioned families were excluded from all priestly duties and privileges. However, Rabbi Yossi concluded from the phrase "most holy things" that they were still permitted to consume simply holy things, due to the presumption of priesthood implied by their previous priestly behavior.^[4] The Talmud debates whether or not a history of eating terumah from outside the Holy Land creates a presumption allowing terumah from the Land of Israel (which is forbidden to non-kohanim by a stronger prohibition, biblical rather than rabbinic); but both opinions accept that the presumption allows the individual to continue eating rabbinic-level terumah.^[4] To summarize: according to Rabbi Yossi, a close reading of the disqualification narrative reveals that the family actually remained qualified to perform certain priestly actions based on their historical behavior.

The rabbis discuss exactly which disqualification might be possessed by this family. The author of Avodah Tamma asserts that they were known to be descended from Aaron, but the suspicion was that one of their ancestors had engaged in a forbidden marriage, and thus disqualified from the priesthood as challalim. The Talmud Yerushalmi explains that Barzilai himself was a Jew, but his daughters were born non-Jewish and converted to Judaism before being three years of age, making their marriage to kohanim forbidden by rabbinic law.^[14] Since the suspected priestly disqualification was only rabbinic, at the conclusion of the investigation this family was permitted to consume terumah.

According to Maimonides, once one is established as a presumptive kohen, the commandment to sanctify the kohen applies to him,^[15] and one should assist him in avoiding the priestly prohibitions. Maimonides considered this presumption to be in force unless a valid objection to his lineage is made before a Beit Din.^[16]

The source of questioning the legitimacy of kohanim is a single responsum of Isaac ben Sheshet, ("Rivash", 1328–1408). In this case, an individual publicly humiliated a kohen. Ben Sheshet was queried whether the humiliator should receive an increased monetary fine, since the humiliated was a kohen. Rabbi Yitzchok responded that "Although it is halacha that he who humiliates another by use of words is not liable, one who embarrasses a Kohen should be publicly rebuked and subject to request forgiveness from and to appease the Kohen — so long as the Kohen exhibits conduct expected from a son of Aaron". He then rejected increasing the penalty in this particular case, since the embarrassed kohen was not a learned kohen, but an unschooled am ha-aretz kohen. Ben Sheshet explained that such a kohen is not entitled to the kohen's extra honor, even if his priestly lineage was verified, and "all the more so today's kohen who possess no lineage document".^[17]

Isaac ben Sheshet's words "today's Kohen who cannot produce his lineage document", written as a sidebar explanation to his final ruling of standard compensation, over time evolved into an underpinning foundation for Poskim who sought to approve rabbinically-questionable marriages to a kohen.

Leading Rabbinic authorities, such as Joseph Trani and Samuel Ashkenazi, disputed reliance on the Isaac ben Sheshet's response for purpose of questioning the authenticity of the kohen. Among other proofs, they cited another response from Isaac ben Sheshet himself^[18] ruling that a kohen who married a divorcee must cease his marriage, proving that Isaac ben Sheshet fundamentally upheld the legitimacy of the presumptive kohen.^[19][20]

• 613 commandments
• Y-chromosomal Aaron