Halakha Ulangan 4:15: Mishneh Torah, Foundations, Mishneh Torah, Prayer and the Priestly Blessing & Gray Matter III

Behold, it is clearly indicated in the Torah and in the Prophets that the Holy One, blessed is He! is Incorporeal for it is said: "That the Lord, He is God in heaven above and upon the earth beneath" (Deut. 4.39); a corporeal being is incapable of being in two places simultaneously; and it is also said: "For ye saw no manner of form" (Ibid.–15); and it is moreover said: "To whom will ye liken me, that I should be equal?" (Isa. 40,25); had He been corporeal He would be like other bodies.

A person who is praying must not break off his prayer, except only if he is in mortal danger. Even when a king who is an Israelite greets him, he must not return the greeting. But if the king is an idolater, he stops, lest the king puts him to death. If, while standing and praying, he sees a king who is an idolater or a man of violence coming towards him, he should shorten the prayer. If unable to do so, he should stop praying. So too, if he sees snakes and scorpions, whose sting in that locality is deadly, moving in his direction, he stops his prayers as soon as they get near him and escapes. But if their sting is not deadly, he does not intermit his prayers.

The question of whether smoking is prohibited not only on Yom Tov but at all times hinges upon the general halachic requirement that we refrain from dangerous and unhealthy activities. The source for this requirement is Devarim 4:15, where we are instructed, “V’nishmartem me’od l’nafshoteichem,” “You shall guard your souls exceedingly carefully.”⁵For a full discussion of this matter, see the essay written by Dr. Shalom Buchbinder and Dr. James DiPoce in The Journal of Halacha and Contemporary Society 42:70-99. The Rambam seems to divide this requirement into two different categories. He lists (Hilchot Dei’ot 4:1) certain foods and activities that, because they weaken the body, one needs (“tzarich”) to avoid, but also mentions (Hilchot Rotzeiach Ushmirat Nefesh 11:5) numerous dangerous activities that Chazal outright prohibited. The activities listed in Hilchot Rotzeiach Ushmirat Nefesh appear to be strictly forbidden, while those in Hilchot Dei’ot seem to be discouraged, but not technically forbidden. Rav Waldenberg (Teshuvot Tzitz Eliezer 15:39) understands that this is because the former are far more dangerous than the latter. Thus, while it is technically prohibited to drink from water in which a snake may have placed its venom, it is not outright prohibited to overindulge in fattening foods.