There are a number of Seforim on the weekly portion of Nitzavim that cite a story in the “Kav HaYashar” regarding the words of the Ramban on Dvarim (29:17), “Ki Mishoresh Matok lo yetzeh mar.”
The book “Kav HaYashar” was first printed in Frankfurt, Germany by Rav Tzvi Hersh Keidenower in 1705. Although it is a well respected Sefer, in this book, the author puts forth an extraordinary claim, regarding a story that he had heard of one of the sons of the Ramban. The story, in essence, has the Pope (presumably Clement the IV) having just read a comment from the Ramban’s recently printed commentary that if a son of a Jew adopted another religion it is a proof that he was never from Jewish lineage in the first place, sent a message to the Ramban. The message stated that since the Ramban’s own son had just converted to Christianity, his interpretation was wrong.
The Kav HaYashar continues that the Ramban was quite saddened until his wife revealed to him that long ago she was attacked by a nobleman on the way to immerse in a faraway place and this particular child was from that encounter. As proof she produced the nobleman’s severed finger which she had bitten off. The Ramban told the Pope what had happened and the Pope brought the nobleman to see his hand. Sure enough after the glove was removed from the nobleman’s hand, the finger was missing.
The story is cited by a number of more contemporary figures other than the Kav HaYashar, including Rav Menashe Klein. Notwithstanding the citations, it is this author’s view that the story is apocryphal for eight different reasons:
- The story is filled with anachronisms. There was no printing press in the time of the Ramban. The story uses the term “Dfus” which indicates that there was an actual physical press and also indicates that the Ramban’s work was printed. This was not the case.
- The Ramban’s three sons, Shlomo, Nachman, and Yehuda are known and had Jewish descendents or interactions. Shlomo had children and the Ramban wrote that they should rather name him after the maternal grandmother and not after him. Nachman was the recipient of letters from the Ramban and authored commentaries. Yehuda is cited It is possible that there was a fourth son, Yoseph, but the record shows that he maintained his Judaism, when he is mentioned. No histories indicate one iota that there was  a descendant of the Ramban or his wife who became Catholic. Also, this is the type of information that would be spread and cannot be kept hidden. The Dominicans, antagonists of the Ramban, would certainly have spread it as certainly as they defamed the Ramban in regard to matters of the disputation.
- Pope Clement IV was never in Aragon, where the Ramban lived.  He lived in Viterbo 50 miles north of Rome throughout his entire pontificate. There is no known personal interaction between the Ramban and Pope Clement IV. The Pope did extend a ban on the Ramban returning to Aragon, but that was by proxy
- The Ramban’s commentary on Chumash was disseminated after he had arrived in Eretz Yisroel. It would not have been possible for the Pope to have read it and interact with the Ramban. The passage that the Ramban makes his statement regarding the issue in in Dvarim (29:14-17). All known histories of the Ramban stat that this was most certainly written while he was in Eretz Yisroel.
- There is no record of any dissemination of the Ramban’s explanations on Chumash other than that which was allowed out at the disputation in Barcelona, where the Pope was not present. After the disputation a ban was placed on the Ramban for two years, which the Pope agreed to extend for longer. Therefore the timeline for such a story or interaction could not have existed.
- It is unlikely that the Ramban’s wife would have obscured such an incident from her husband and have kept the severed finger wrapped up for so many years, until the child grew up. Had the incident have been historical, she would have shown it to him while the finger was still fresh.
- It is unlikely that the Ramban would have “been more troubled over his incorrect interpretation” than having a son who would have converted to Catholicism.
- It is extremely unlikely that this information would survive in oral form only from the 500 year period and not have been written down in the interim. Furthermore, this story would have had to make the trip from Aragon Spain to Frankfurt Au Man, Germany. There is no trace of this story in writing during these five centuries.
The author can be reached at yairhoffman2@gmail.com
Really Rabbi Hoffman: Do you need to spend so much effort to disprove an obviously absurd story?
>The story, in essence, has the Pope (presumably Clement the IV) having just read a comment from the Ramban’s recently printed commentary
Not printed.
Dr. Shnayer Leiman, during one his monthly shiurim at his home debunked this story also. Shkoyach!