Sources: Talmud Bavli Yoma 85b; Sanhedrin 74a; Leviticus 18:5 (‘live by them — not die by them’); Ezekiel 20:11; Maimonides, Laws of Shabbat 2:1–3; Shulchan Aruch, Orach Chaim 328.

Pikuach nefesh (preservation of life) is the halakhic principle that the obligation to save human life overrides virtually every other commandment in the Torah. Yoma 85b is the locus classicus: R. Shimon ben Menasya derives from Leviticus 18:5 (‘You shall keep my statutes and ordinances which a person shall do and live by them’) that one should ‘live by them — and not die by them.’ Violating the Shabbat to save a life is not merely permitted but required. Sanhedrin 74a establishes the three exceptions: even to save one’s life, one may not commit murder, sexual immorality, or idolatry. These three are yehareg ve-al ya’avor — be killed rather than transgress. Everything else yields to life. The theological depth: this principle establishes a hierarchy that places the preservation of God’s image (the human person, created b’tzelem Elohim) at the apex of the halakhic system. The Mishnah in Sanhedrin 4:5 expresses it: ‘Whoever saves a single life, Scripture accounts it as if he had saved an entire world.’ Practical implications run throughout Jewish law: any Shabbat restriction, any fast, any dietary law yields to medical necessity. This makes Jewish medical ethics distinctive — the obligation to heal is not merely permitted but commanded, and the physician who heals on Shabbat is not violating Shabbat but fulfilling a mitzvah. Modern bioethical debates in Jewish law (organ donation, end-of-life care, triage) are governed by pikuach nefesh’s priority structure.