There is a positive commandment to fast on Yom Kippur, as we read, “In the seventh month, on the tenth day of the month, you shall deprive (ve-initem) yourselves” (Vayikra 16:29). The primary expression of this deprivation (inui, which may also be translated “affliction” or “suffering”) is refraining from life-sustaining food and drink, and the punishment of karet (extirpation) in the case of a knowing transgression and a sin offering in the case of an unknowing transgression apply only to one who eats or drinks. Nevertheless, the mitzva of inui includes four additional prohibitions, all of which are forms of deprivation. Together with the prohibition on eating and drinking, there are a total of five prohibitions: a) eating and drinking; b) washing; c) anointing; d) wearing shoes; e) sexual relations (Yoma 73a).
The mitzva of inui does not require us to do things that inflict pain, like sitting in the midday sun. Rather, the mitzva is to desist from certain things whose deprivation causes suffering (Yoma 74b and 76b-77b). The basis for this understanding is the verse, “It shall be a Shabbat (Shabbat) of complete rest (Shabbaton) for you, and you shall deprive yourselves” (Vayikra 23:32). Our Sages expound: “Shabbat” – you should refrain (tishbetu) from eating and drinking; “Shabbaton” – you should refrain from other activities that would reduce inui (Yoma 74a). The Sages also infer from the fact that the Torah commands us to “deprive ourselves” five times that there are five activities from which one must desist.
The poskim disagree about how severe the additional four prohibitions are. Some say that since the Torah never explicitly states that eating and drinking are prohibited, but rather states, “You shall deprive yourselves,” it means that all five deprivations are included in the Torah’s commandment. According to most poskim, however, only eating and drinking are prohibited by the Torah, because the primary expression of inui is to be deprived of them. Still, the Torah did not explicitly state that the mitzva is to refrain from eating and drinking, but commanded us to deprive ourselves, to teach us that there must be additional expressions of our deprivation. Based on this, the Sages enacted the other four prohibitions.
What do these five deprivations correspond to? R. Ḥisda says: They correspond to the five times that the Torah mentions “inui”: “On the tenth [day of the same seventh month, you shall observe a sacred occasion when you shall deprive yourselves” (Bamidbar 29:7)]; “But the tenth [day of this seventh month is the Day of Atonement. It shall be a sacred occasion for you; you shall deprive yourselves” (Vayikra 23:27); “It shall be] a Shabbat of complete rest [for you, and you shall deprive yourselves” (ibid. v. 32); “It shall be] a Shabbat of complete rest [for you, and you shall deprive yourselves; it is a law for all time” (ibid. 16:31)]; “And this shall be to you [a law for all time: In the seventh month, on the tenth day of the month, you shall deprive yourselves” (ibid. v. 29).]
It would seem from this exegesis and that of “Shabbat Shabbaton” (Yoma 74a) that all five prohibitions are from the Torah. Indeed, this is the position of She’iltot, Behag, Itur, and Yere’im.
However, other Rishonim maintain that the additional four afflictions are rabbinic. When the Torah speaks of the punishment of karet in the context of Yom Kippur (Vayikra 23:30), it says, “I will cause that person to perish (ve-ha’avadeti et ha-nefesh).” The Sages explain that liability to punishment by karet is limited to the type of deprivation that would lead to death (ibud nefesh) if continued for an extended period (Yoma 74b). This implies that it is only eating and drinking that are prohibited by the Torah. Furthermore, we find that R. Eliezer permits a king and a new bride to wash their faces and permits a postpartum woman to wear shoes. Moreover, washing minors and applying cream to them is permitted. If these actions were prohibited by the Torah, we would not find these leniencies. For these reasons, Rabbeinu Tam, Ri, Riva, Rashba, Rosh, Ritva, Me’iri, and Sefer Ha-ḥinukh maintain that the additional four prohibitions are rabbinic, and the prooftexts simply support the rabbinic laws (“asmakhta”) rather than serve as their source. Those who nevertheless maintain that all the deprivations are Torah prohibitions explain that the Torah authorizes the Sages to delineate their parameters. Therefore, the Sages can choose to be lenient in certain cases (Ran). If the additional deprivations are from the Torah, why do they not carry the punishment of karet? Because as long as one fasts, he is observing the primary deprivation.
I would like to suggest that all agree that these four prohibitions are rooted in the Torah, while the details are rabbinic. This is why the Torah obliquely describes the mitzva of the day as “inui,” which includes all types of deprivation, not just eating and drinking. According to most poskim, the Torah requires us to desist from eating and drinking (the primary form of deprivation), and leaves other matters which have an aspect of deprivation to the discretion of the Sages, while alluding in the verses that there are grounds to prohibit five things. Others say that even the four additional prohibitions are absolutely included in the Torah commandment, for desisting from them is part of the mitzva of inui. However, since they are not the primary forms of inui, their parameters were given to the Sages to delineate.