The measurement of “k’zayit” is frequently used in reference to prohibitions of the Torah involving eating; i.e., eating less than a k’zayit does not make one eligible for punishment. A prohibition, though, exists: according to Reish Lakish, a rabbinical one, and according to R. Yochanan, who the halakhah follows, a biblical one (Yoma 73b). Several theories exist in the literature of the acharonim to explain the dispute between Reish Lakish and R. Yochanan, many of them addressing the nature of the relationship between a prohibition and its measurement.

One suggestion is the issue has to do with whether biblical prohibitions are understood to be inherently negative (issurei cheftzah) or defined by the prohibition on the individual interacting with them (issurei gavra). If the former is accurate, it could be stated that the measurement is only relevant toward determining the culpability of the individual, while the object, inherently prohibited, maintains this status in any quantity. Alternatively, if the prohibition is individual-based, being that the act of eating is defined as a k’zayit or more, any less than that is completely out of the scope of prohibition, and thus any injunction must be rabbinic in origin. (See Tziyunim L’Torah, 22; Resp.Maharash, petichah to Vol. III, #4; Resp. Kol Aryeh, 77, and others).

Others suggest the issue involves the view, held by the R’em, that on a straight biblical level, “eating” refers to any amount; the Halakhah l’Moshe MiSinai redefines it for the purposes of punishment, but the transgression itself is there at any quantity. This would correlate to the position of R. Yochanan. The Mishneh L’Melekh (Hil. Chametz U’Matzah, 1:7) disagrees with the R’em, and believes the definition of the act of eating, on a Torah level, to be a k’zayit. This would match the view of Reish Lakish, that any transgression involved must be rabbinic. In other words, as expressed by R. Ben Tziyon Uziel (Resp. Mishpetei Ouziel, II, 12) the dispute between the R’em and the Mishneh L’Melekh is as to what is the effect of the Halakhah L’Moshe MiSinai: to define the measurement of punishment, or to define eating itself (see also Resp. Maharash, IV, 2; Resp. Da’at Sofer, O.C. 107; Resp. Chikrei Lev, O.C. 58; Ohel Moshe, 1; Zivchei Shlomo (Chiddushei Sugyot, 11; Birkat Yehudah, 18).

Another suggestion is the issue is whether or not we are permitted to apply Biblical law based on our understanding of its reasoning (darshinan ta’ama d’kra). Normally such latitude would indicate a lenient application; in this case, it would tend toward stringency, as R. Yochanan’s position is accompanied by the logical statement “kevan d’chazei l’itztarufei, issura ka achil” (since a full quantity is prohibited, any amount is forbidden), applying that logic would lead to a biblical prohibition covering any quantity. (See Korban Tziyon to Temurah, Responsa, 3).

R. Avraham Loftiber (Zera Avraham, II, 51) notes that there is actually a dispute in the Talmud Yerushalmi (Peah 1:1) as to whether the measurements are Halakhah L’Moshe MiSinai or rabbinic in origin, and that Resh Lakish may take the latter view. In his commentary to R. Loftiber’s work, R. Yitzchak Blazer (Ha’arot Pri Yitzchak) explains the intent of this comment as based on the concern that a later Beit Din may teach that the understanding of a k’zayit is actually less than it appears to us at present. As Halakhah L’Moshe MiSinai is treated stringently when circumstances are indeterminate [a disputed point; see Resp. Noda B’Yehudah. II, Y.D. 146], we must factor that possibility into behavior. However, if measurements are rabbinic in origin, the rule of leniency in that area would permit this concern to be dismissed.