A central observance of Yom Kippur—fasting, which we will need to define more fully—is codified by the Torah in both a positive and a prohibitive form (an aseh and a lo ta`aseh). That means that, aside from defining the mitsvah itself, we should spend some time considering why Hashem would choose to phrase these obligation in both ways. In fact, it is the positive expression that is the more puzzling, since we would instinctively think of this as a prohibition (don’t eat, etc.).

Before we consider that general question, we should define the obligation. Colloquially, we call inui nefesh "fasting," but the Mishnah in the beginning of the eighth chapter of Yoma lists five forms of soul-affliction, eating and drinking (which count as one), washing, anointing with oil, wearing shoes, and engaging in marital relations. Eating and drinking , which count as one in terms of the five inuyim), are the only ones that incur karet for one who violates it.

Most eating-obligations focus on a kezayit, an olive’s worth, or a ke-bezah, the volume of an egg. Drinking obligations generally focus on a rov revi`it, but neither of those apply to Yom Kippur. While any amount of eating or drinking is prohibited, because we rule that hatsi shiur assur min haTorah--Torah prohibitions extend to even less than the shiur— knowing the shiur helps us in three ways. First, as mentioned, that is the amount that will incur the highest level of punishment. Second, if someone falls ill on Yom Kippur and needs to eat, we would first (if there is no danger in trying to do so) try to feed them less than that amount, so as to minimize the extent to which the person has to violate Yom Kippur. Third, the shiur gives us some insight into what the Torah was aiming for in prohibiting eating and drinking on this day.

Had the shiur been a kezayit, we would have understood that the Torah wished to prohibit food on this day, as pig is always prohibited, and as hamets is prohibited on Pesah. Instead, the gemara records a shiur of a large date (and the volume of one cheek for drinking), the point at which food puts a person’s soul at ease. That person can then no longer be described as being engaged in inui nefesh, in afflicting his or her soul. To understand why the Torah would care about that state, we turn to Rambam and Sefer haHinukh. In Mitsvah 313, the Hinukh sees inui nefesh as simply a way to insure that we are not distracted by our connection to the physical. Since the day has such lofty goals, allowing our ordinary physical needs and interests to interfere would endanger our success at the broader goals of teshuvah, and of gaining forgiveness.

Rambam seems to say that we build towards inui nefesh, soul affliction, rather than engage in it. In explaining why the Torah uses the word shabbaton regarding the act of inui, Rambam says that we are supposed to refrain from these actions (or rest from them) until we reach inui. That reading suggests that when we refrain from these actions on Yom Kippur night (after a large seudah mafseket), we are not yet afflicting our souls. Rather, we are setting the stage for soul affliction, a state that may only be reached towards the late afternoon of Yom Kippur. Only then, when the soul feels afflicted, have we achieved inui nefesh.

Rambam’s view captures an important element of our Yom Kippur experiene, in that our inuy deepens over the course of the day— an hour after Yom Kippur starts, we have just eaten, washed, and am not yet uncomfortable for lack of shoes; it is only over time that our souls become afflicted by those lacks. Rambam’s view also indicates that the inuyim themselves are not the problem, but that indulging them prevents us from having another experience. That view of inui nefesh as a state to be achieved could also explain why the Torah would make this a positive commandment as well as a prohibition. Were we only concerned about distraction, the Torah could have just prohibited these activities. To the extent that the inyuyim help us achieve a certain state, we understand why it was a positive obligation as well. Rambam does not define the desired soul-state more specifically than this, but trying to understand that state might enrich our observance of the day.

The other inuyyim are of a clearly different status than eating and drinking. Many rishonim thought they were rabbinic, for two reasons. First, Hazal treat these inuyim differently, allowing actions that seem like the inui itself for various reasons. For example, Hazal allowed washing off actual dirt from one’s body and also allowed walking through water for various reasons. If the requirement to avoid wetness was de-oraita, the reasoning goes, Hazal would not have been so lenient. Second, the gemara explicitly categorizes the prooftexts for the inuyim other than eating/drinking as asmakhta, meaning they are not offered as definitive proof, but as a textual support. Generally, if a rule is derived through an asmakhta, we think of that rule as de-rabanan.

Others, however (and the aharonim debate which view Rambam held), assert that all the inuyim are de-oraita, but that the Torah granted Hazal the right to define those other than eating. In other words, Hashem only told us to afflict our souls on Yom Kippur, with that command definitely including the obligation to refrain from food and drink. Beyond that, Hashem expected Hazal to define other activities that could support that same experience, which Hazal defined as washing, anointing, etc. This concept of Hazal’s contribution to the definition of Torah law, which also appears regarding the prohibitions of work on Hol haMoed, assumes that they were given a role in completing the process of defining central Judaism, not only in adding their own protective boundaries and obligations as time went on. With Best Wishes for a Gemar Hatimah Tovah, Shabbat Shalom.