The Untold Story of R’ Mordechai, the Alter Rebbe’s Brother

Photo: Handwriting of the Alter Rebbe, City of Kopust

This Shabbos, the 11th of Adar, marks the 200th yahrzeit of R’ Mordechai Posner, the younger brother of the Alter Rebbe who served as the Rav of Orsha, White Russia.

By Rabbi Mordechai Rubin, Colonie NY – Colonie Chabad

I am grateful to a few relatives for their assistance in the preparation of this article, as well as to the information in the “Teshura” on (our relative) R’ Sholom Ber Pewzner and his family roots, published for the Bar Mitzvah of Berl Pewzner, 17 Sivan 5782. 

This Shabbos, the 11th of Adar, marks 200 years (5583-5783;1823-2023) since the yahrtzeit of R’ Mordechai Posner, the Rav of Orsha, White Russia. He is buried in Kopust.

I am referring to R’ Mordechai, one of the brothers of the Alter Rebbe. Many are familiar with his more famous brother R’ Yehudah Leib (author of “She’eris Yehudah”), the great halachist who was involved in the Alter Rebbe’s Shulchan Aruch. They had two additional brothers, R’ Mordechai & R’ Moshe (R’ Mordechai was the third son, born following the Alter Rebbe and R’ Yehudah Leib, and before R’ Moshe).

[Their family name “Posner”, or “Pozner”, originated from the Polish city of “Posen”, as do many others with the same or similar names (such as Posen or Poznansky). The Alter Rebbe’s paternal great-grandfather, R’ Moshe Posner, who was a great Talmid Chacham as well as a successful Shtadlan (advocate) for the Jewish community, served as the Rosh HaKahal of Posen.]

In fact, many prominent large families of Chabad Chassidim whose roots come from the Alter Rebbe’s family, aside from the Alter Rebbe’s direct descendants – Beis Harav – are in fact descendents of R’ Mordechai: The Posners, Pevzners and many other prominent families of Chabad Chassidim.

I was curious to learn more about him, as I am his descendant on my mother’s side and that’s how I am distantly related to my wife’s family. In addition, my birthday, the 10th of Adar, is close to his yahrtzeit on the 11th day of the month, and also share his namesake Mordechai, so I was intrigued to learn more about him.

After a bit of research, and was myself fascinated by the once-mysterious R’ Mordechai who I descended from and who connected me somewhat to Beis Harav, I figured that it may be also beneficial to share my findings with others. 

There are a number of references in the Frierdiker Rebbe’s writings about R’ Mordechai, and as of yet there is not much available in an English “one stop” essay format about him. Accordingly, I have put together the following as we mark his 200th yahrzeit this Shabbos. 


Firstly, he was known as a great Gaon in Torah, reviewing the Gemara numerous times and then expounding on it with original insights. 

In the early days of Chabad, he would live in Liozna alongside the Alter Rebbe, and later, he would serve as Rov in Orsha. Although surely incomparable to the Alter Rebbe, the Frierdiker Rebbe describes Reb Mordechai with great praise, among them titling him HaRav HaKodesh. In the inception of the Alter Rebbe’s leadership, he was instrumental in the Chadorim — the special advanced Yeshivos — which the Alter Rebbe set up. There, his focus was on the in-depth and organized teaching of Nigleh and in the administration of tests on the subject matter.

Chassidim who wished to see the Alter Rebbe for Yechidus, would go through a few weeks’ time of learning locally in the Alter Rebbe’s town of Liozna. Afterwards, R’ Mordechai and his brother R’ Moshe would test them on this study “Seder”, before they would subsequently be allowed to see the Alter Rebbe for Yechidus. Here is an excerpt from a Sicha of the Frierdiker Rebbe (Sichas Shabbos Shemos, 21 Teves 5702):

“Each young man was then given a study schedule in Nigleh and in Chassidus, and only after his prescribed two or three weeks, was he admitted to the Rebbe’s study for Yechidus. The examiners were the Rebbe’s brothers – R. Mordechai, whose expertise was the Talmud Bavli with the classic commentaries of the Rishonim, and R. Moshe, whose expertise was the Talmud Yerushalmi and the writings of Rambam. At times they examined separately and at times, jointly.”

In the recount of the famed Chossid R’ Baruch Mordechai of Bobroisk: 

“I had spent those few weeks in the company of three veritable princes. I had observed how reverently R. Mordechai related to every word written by Rashi, and how earnestly he weighed every query and answer proposed by Tosafos. I had heard the subtle logic of the arguments with which R. Moshe analyzed a statement of Rambam, or resolved seeming contradictions between parallel passages in the Yerushalmi and the Bavli. I had seen how R. Leib, seeking to deduce valid halachic conclusions, shuddered with an awe of Heaven as he took note of a gloss written by Rama or a fine point made by the Taz or Shach. Those experiences, and above all the Rebbe, had opened up a fresh conduit of progress in my comprehension and mastery of the Torah, and it was with a truly broken heart that I took my leave of them all.”

The Tzemach Tzedek described his great-uncle R’ Mordechai as being tremendously sharp and a master of deep, yet straightforward thinking. His multi-tiered layers of explanation were always especially clear and pleasant. After the Alter Rebbe’s passing (5573/1812), R’ Mordechai worked alongside his brother the Maharil in reviewing the Alter Rebbe’s Shulchan Aruch in preparation for print, as seen in the  “Introduction” to the Alter Rebbe’s Shulchan Aruch, by  his three sons. Reb Mordechai passed away ten years after the Alter Rebbe, 5583/1823.

Reb Mordechai and his wife had at least one son and two daughters. The son was named Moshe Dov Ber, which is speculated to be for Reb Mordechai’s grandfather, Reb Moshe of Posen, and for the Maggid of Mezritch. Interestingly, this seems to identify Reb Mordechai as a talmid or chassid of the Maggid in addition to his hiskashrus to his brother, the Alter Rebbe.

One of the daughters, whose name is unknown, married a Reb Shimon Lifshitz, and their son was the Chassidisher Gaon, Reb Dovber Lifshitz. The Rov of Vietka, Belarus, he is known for his sefer “Gulos Iliyos” on the laws of Mikvah. The second daughter (perhaps Basya) became the matriarch of the Levertov family.

[The information on their subsequent generations is beyond the scope of this article. On some of those descendants and their families – see “Teshura” of Bar Mitzvah of Berl Pewzner, 17 Sivan 5782]. 

I had wondered, being as Chassidus was new at the time, and not all of the Alter Rebbe’s relatives embraced it did R’ Mordechai have an appreciation for Chassidus in addition to his genius in Nigleh & respect for the Alter Rebbe in Torah? Well, there is a record of him attending the Great Debate in Minsk, where he commented on how he appreciated the Alter Rebbe’s profound responses to those that opposed Chassidus.

In fact at the very first celebration of Yud Tes Kislev the Frierdiker Rebbe relates (Shabbat Chol Hamoed Pesach 5700 (1940) ), that the Alter Rebbe entered the room flanked by his brother R’ Yehudah Leib on the right & Reb Mordechai on his left. Following this first line were other prominent Chassidim & members of the Beis Harav.  

We also can see the actual handwriting, ksav yad kodesh, of the Alter Rebbe mentioning his brother’s name in a letter regarding the printing press of the Slavita Shas, where R’ Mordechai served as a Shliach of the Alter Rebbe along with R’ Sholom Shachne the father of the Tzemach Tzedek.

I would like to end with a fascinating account, a story that took place in R’ Mordechai’s youth along with his brother the Alter Rebbe. Interestingly, it is recorded in the very famous Sicha of Kol Hayotzei – Erev R”C Tammuz 5702 :

At that time R’ Baruch was living in the estate which he had received as his dowry, some three viorsts from Liozna. His gifted six-year-old son was once sitting in his father’s orchard with his brother, R’ Mordechai, who at the time was five years old, and as they studied Chumash together they came to the verse, “These are the descendants of Seir, the Chorites, the dwellers of the land.”

Rashi explains that they are so described because they were experts in agriculture; by tasting the soil they could tell which crops should be planted in each spot. The young R’ Mordechai found this hard to understand. Does not all soil look the same? The Alter Rebbe explained that there are ways of knowing what goes on beneath the surface. “For example,” he told his brother, “water flowing underground can be heard. Under that mound of white stones over there, there flows a powerful stream of water.”

The stream eventually broke through the mound and formed a pool, which proved very beneficial to the farmers of the surrounding villages when a cattle plague broke out a few years later. From that time on, they called this estate (in Russian) “the white fountain,” which is also the translation of the name (in Yiddish) by which R’ Baruch became known. 

At a later occasion, the Frierdiker Rebbe alluded to this story (Lag B’Omer 5704)

Once, after explaining a certain concept to his brother, R. Mordechai, the Alter Rebbe told him that if he put his ear to a certain stone, he would hear a wellspring under it. Some time later the spring was discovered. It became known as Bieli Rutchei (“White Brook”), and those who drank of its waters were healed of an epidemic brought on by a local malevolent gentile with supernatural powers. ** See Kuntreis Erev R”C Tammuz, 5702 [1942], p. 29.

We once traveled there to see the estate, but there was no sign left except for stones. 

Many of us have heard the later part of the story of the Alter Rebbe and the spring. But to me, this account of the background of his first encounter with the “spring” with R’ Mordechai served to be novel and refreshing. 

This story of the Alter Rebbe in his youth, speaks to his mission in life, seeing & finding the Mayim Chayim   ‘living waters’ in Torah & Yiddishkeit where others saw the stones that covered it up. The Inner dimension of Torah always sees the living water and life in what others seem as lifeless and inanimate. 

The Alter Rebbe’s vision in finding that spring proved to be literally lifesaving at the time, and so this approach of Pnimius HaTorah continues to be life-saving till today.

The lessons from this story can be much further elaborated & expounded upon, but I will leave that to you, to find the living waters beneath the stones.

In keeping in line with the Rabbonim's policies for websites, we do not allow comments. However, our Rabbonim have approved of including input on articles of substance (Torah, history, memories etc.)

We appreciate your feedback. If you have any additional information to contribute to this article, it will be added below.

Leave a Comment

Your email address will not be published. Required fields are marked *

advertise package