One of the stories that recalls Rabbi Moshe Tendler’s charming candor starts out like a timeworn Jewish joke.

Three rabbis — one Reform, one Conservative and one Orthodox — were asked to give their wisdom on sex therapy at a conference in 1985 at Mount Sinai Hospital in Manhattan. The Reform rabbi concentrated on the sanctity and sensitivity with which sex must be treated, and the Conservative rabbi fairly matched him in lofty terms. But the Orthodox rabbi, Moshe Tendler, got down and dirty, as he himself would never have said.

He talked with vivid anatomical precision about what acts and positions could be used in sex therapy as permitted in Jewish law. Men, for example, could not masturbate, he told the crowd, because of the biblical injunction against the wasting of seed, but women may do so as a therapeutic technique since there was no written prohibition.

“A marriage without sexuality is a weak marriage,” Rabbi Tendler told the audience, adding that every sex act should give maximum pleasure to both spouses.

It was just one pronouncement that made Rabbi Tendler a singular authority on medical ethics in the Jewish world, in no small part because he was both a master of the Torah and Talmud and a trained microbiologist. His rulings on the definition of death had particular resonance.

He died on Sept. 28 at 95 at a hospital in Rochelle Park, N.J. He lived in the largely Orthodox hamlet of Monsey, NY., in Rockland County.

A son, Rabbi Mordecai Tendler, said his father had suffered a series of bacterial infections after being hospitalized for a broken leg, noting that in his eulogy for his father he had remarked on the irony that a microbiologist should die this way.

In articles, three books, speeches and frequent legislative testimony, Rabbi Tendler made influential statements on a number of controversial issues — the clinical definition of death, organ transplantation, permissible circumcision techniques, stem cell research and more — that shaped how the larger Jewish community addressed these questions.

He was appointed the posek — a decisor of, among other matters, how Jewish law should respond to advances in science and technology — for the Association of Orthodox Jewish Scientists and brought to that title a singular résumé: He was ordained a rabbi by Yeshiva University in 1949 and earned a doctorate in microbiology from Columbia University in 1957. It did not hurt that his father-in-law was Rabbi Moshe Feinstein, the greatest posek of his age, who often relied on Rabbi Tendler’s expertise in biology to shape his own thinking.

Rabbi Tendler was most famous in medical ethics circles for his ruling in the late 1970s that the complete cessation of brain functioning rather than of the heartbeat constituted death.

At the time, most authorities believed that death occurred when a heart stopped. But as respirators could keep a heart pumping when the brain no longer functioned, another definition seemed necessary. Advances in transplantation — particularly of the heart itself — rendered a decision urgent.

Rabbi Tendler based his decision strictly on the physiology of the body and so chose rather blunt terms to emphasize his reasoning, saying a person whose brain ceased was “physiologically decapitated.”

When a group of rabbis insisted that it was the cessation of the heartbeat that constituted death, he brusquely dismissed their arguments.

“I believe you’re ignorant on this topic,” he quoted himself as telling the rabbis in an interview with the Jewish Telegraphic Agency. “That’s not an insult. It’s a fact.”

His ruling opened the way for observant Jews to donate and receive organs for transplanting before a heart gave out, said Dr. Edward R. Burns, who is executive dean of the Albert Einstein College of Medicine in the Bronx and who studied both biology and Talmud with Rabbi Tendler at Yeshiva University.

Rabbi Tendler’s perspective also shaped New York State laws and regulations governing transplants, Dr. Burns said, and was imitated by other states as well.

More recently, Rabbi Tendler backed New York City health officials in trying to ban a circumcision procedure in which the mohel — a Jew trained in the rite — sucks blood from an infant’s wound with the lips. Officials told resistant mohels and their Hasidic and other ultra-Orthodox supporters that the procedure put babies at risk of a possibly fatal herpes infection. Rabbi Tendler advised the mohels to do the procedure with a tube. City officials ultimately allowed the traditional procedure if parents consented, but stopped requiring a signed consent form.

In the early 2000s he challenged the administration of George W. Bush when it tried to limit federal funding for research into stem cells, which are harvested from embryos remaining after an in vitro fertilization. Rabbi Tendler did not believe that the embryos were full-fledged persons, basing his interpretation on passages in the Torah and Talmud regarding the damages that must be paid when someone causes a woman to miscarry before the 40th day of her pregnancy.

Alan Jotkowitz, a professor at Ben Gurion University of the Negev specializing in medical ethics, told the Jewish Telegraphic Agency that Rabbi Tendler “saw no conflict between scientific knowledge and Torah.”

“He said you could see God’s wisdom in the Torah, but Rabbi Tendler also thought you could see God’s wisdom in nature and studying nature,” Professor Jotkowitz said.

Moshe David Tendler was born on Aug. 7, 1926, on the Lower East Side of Manhattan, one of seven children. His father, Rabbi Isaac Tendler, an immigrant from Lithuania, was the head of the Rabbi Jacob Joseph Yeshiva in the neighborhood and the rabbi of the Kominitzer Synagogue. Moshe attended the yeshiva through elementary school. His mother, Bella (Baumrind) Tendler, was a real estate lawyer.

Rabbi Tendler’s more than 80-year association with Yeshiva University began when he entered its Talmudical Academy, a high school. He later took up rabbinical studies at the university’s Rabbi Isaac Elchanan Theological Seminary. One of his teachers was the vaunted Talmudist and philosopher Rabbi Joseph Soloveitchik, with whom he studied for seven years.

Drawn to biology from childhood, Rabbi Tendler, while studying at Yeshiva, simultaneously took evening courses at New York University. There he received a bachelor’s degree in biology and then a master’s degree in the subject before eventually earning the doctorate at Columbia.

Yeshiva hired him as a biology instructor in 1952, and within a few years he was appointed assistant dean in charge of student affairs. He was later named a rosh yeshiva at the seminary, a title given to the school’s leading teachers and one he held at his death. He lectured from his hospital bed by Zoom until five months ago, his son Mordecai said.

Years ago as a student, Rabbi Tendler was studying at a local library when he was approached by Shifra Feinstein, Rabbi Feinstein’s daughter, who asked him a chemistry question. They went on to marry and have eight children, moving to Monsey in 1960. There he was named rabbi of the Community Synagogue.

Mrs. Tendler died n 2007. Beside Mordecai, he is survived by three daughters, Rivka Rappaport, dean of an Israeli high school, Sara Oren, a nurse at Hadassah Medical Center in Israel, and Ruth Fried, chair of science at Yeshiva University High School for Girls in Holliswood, Queens; four other sons, Yacov, an internist, Aron, a rabbi in Baltimore, Hillel, a Baltimore lawyer, and Eli Don, a lawyer on Long Island; a brother, Sholom, dean of a yeshiva in Los Angeles; more than 200 grandchildren and great-grandchildren; and four great-great grandchildren.

In a tribute he wrote for the online magazine Tradition, Dr. Edward Reichman, a professor of emergency medicine at Einstein and a former student of Rabbi Tendler, said that Rabbi Tendler had taught generations of observant medical students to think about the implications of Jewish ethics in their training and that he had “forever changed the way the Jewish world analyzes and integrates the field of medicine through the lens of Torah.”

“It is no exaggeration,” he added, “to say that Rabbi Tendler’s name was in the Rolodex (or, today, smartphone) of every religious Jewish physician.”