ואליבא דרבי עקיבא דאמר אדם אוסר עצמו בכל שהוא
and the mishna is in accordance with the opinion of Rabbi Akiva, who said: A man prohibits himself from any amount. If a man swears that he will not eat, he thereby prohibits himself from eating even the smallest amount of food. Therefore, Reish Lakish himself maintains that eating a half-measure does not constitute a prohibition.
וכי תימא כיון דאית ליה היתר מן התורה קא חייל קרבן שבועה והתנן שבועת העדות אינה נוהגת אלא בראויין להעיד והוינן בה למעוטי מאי רב פפא אמר למעוטי מלך
And if you say: Perhaps Reish Lakish maintains that since a half-measure is permitted by Torah law, despite the fact that it is prohibited by rabbinic law, one is liable to bring an offering for violating an oath, then there is the following problem: Didn’t we learn in a mishna: An oath of testimony where one is sworn to give testimony on something that he saw or knew, applies only to those who are eligible to give testimony. If one who is ineligible to testify swears an oath to give testimony, the oath is invalid even if he does not testify. And we discussed it: The statement: Those who are eligible to give testimony, comes to exclude what? After all, it was already said that the oath does not apply to women, relatives, and other disqualified people. Rav Pappa said: It comes to exclude a king. A king is not disqualified from giving testimony, but he does not testify before a court, due to the requirement to give respect to a king.
רב אחא בר יעקב אמר למעוטי משחק בקוביא והא משחק בקוביא מדאורייתא מיחזי חזי ורבנן הוא דפסלוהו ולא קא חיילא עליה שבועה
Rav Aḥa bar Ya’akov said: It comes to exclude one who plays with dice, whom the Sages disqualified from giving testimony. But surely one who plays with dice is eligible by Torah law to give testimony, and it is the Sages who disqualified him. Despite this, an oath of testimony does not apply to him by Torah law, even though the prohibition on his testifying is rabbinic.
שאני התם דאמר קרא (ויקרא ה, א) אם לא יגיד והאי לאו בר הגדה הוא כלל
The Gemara rejects this by distinguishing between the two cases: It is different there, in the case of testimony, where the verse states: “If he does not utter it, then he shall bear his iniquity” (Leviticus 5:1), i.e., a man who can testify but doesn’t do so should be punished. But this person cannot ever give testimony since the court will not accept his testimony. The Torah makes liability for an oath of testimony contingent on one’s ability to testify. Therefore, an oath of testimony would not apply to someone unable to testify. However, one who takes an oath not to eat is liable if he breaks that oath, notwithstanding the rabbinic prohibition against eating less than a measure of forbidden food. Consequently, this rejection does not stand, and the first explanation remains.
וכל היכא דתני ענוש כרת לא תני אסור והתניא אע"פ שאמרו אסור בכולן לא אמרו ענוש כרת אלא על האוכל ושותה ועושה מלאכה בלבד הכי קאמר כשאמרו אסור לא אמרו אלא בכחצי שיעור אבל כשיעור ענוש כרת ואף על פי שענוש כרת אין ענוש כרת אלא אוכל ושותה ועושה מלאכה בלבד
§ The Gemara’s initial assumption is that the mishna’s use of the word prohibited is referring to a transgression not punishable by karet. The Gemara asks: And anywhere that it teaches that transgressing is punishable by karet, does it never teach using the word prohibited? Was it not taught in a baraita: Although they said the word prohibited with all of the five Yom Kippur afflictions, they said that the punishment of karet applies only to one who eats, or drinks, or performs prohibited labor. This means that the word prohibit is used with transgressions punishable by karet as well. The Gemara rejects this. This is what the baraita is saying: When they said that those five activities are prohibited, they said that only with regard to a half-measure; but a full measure is punishable by karet. And although a violation is punishable by karet, it is punishable by karet only if one eats, or drinks, or performs prohibited labor; these alone are the cases where karet is incurred.
ואב"א כי קתני אסור אשארא דתנו רבה ורב יוסף בשאר סיפרי דבי רב מניין ליוה"כ שאסור ברחיצה בסיכה ובנעילת הסנדל ובתשמיש המטה ת"ל (ויקרא טז, לא) שבתון שבות
And if you wish, say instead that when it is taught in the mishna using the language of prohibited, it is referring to the other transgressions, which do not incur karet. As Rabba and Rav Yosef taught this in other books of Rav’s school, i.e., the Sifrei, the halakhic midrash on Numbers and Deuteronomy: From where is it derived that it is prohibited to engage in bathing, and in smearing oil on one’s body, and in wearing shoes, and in having relations on Yom Kippur? The verse states: “Shabbaton” (Leviticus 16:31), meaning resting and refraining from certain activities. Therefore, the word prohibit is used with these activities, but they are not punishable by karet.
גופא חצי שיעור רבי יוחנן אמר אסור מן התורה ריש לקיש אמר מותר מן התורה רבי יוחנן אמר אסור מן התורה כיון דחזי לאיצטרופי איסורא קא אכיל ריש לקיש אמר מותר מן התורה אכילה אמר רחמנא וליכא
§ Apropos the dispute between Rabbi Yoḥanan and Reish Lakish, the Gemara deals with the matter itself: What is the law with regard to a half-measure? Rabbi Yoḥanan said: It is prohibited by Torah law. Reish Lakish said: It is permitted by Torah law. The Gemara elaborates: Rabbi Yoḥanan said it is prohibited by Torah law because it is fit to combine with another half-measure. If one continues to eat more, he will eat a whole measure, which is punishable by Torah law. Therefore, even when he eats the first half-measure he is eating forbidden food. Reish Lakish said it is permitted by Torah law. His reason is as follows: With regard to all forbidden foods, the Merciful One states in the Torah: “Eat,” for example in the verse: “You shall eat neither fat nor blood” (Leviticus 3:17). Eating is defined as consuming a minimum of an olive-bulk, and there is no prohibition if one eats less than an olive-bulk.
איתיביה ר' יוחנן לריש לקיש אין לי אלא כל שישנו בעונש ישנו באזהרה כוי וחצי שיעור הואיל ואינו בעונש יכול אינו באזהרה ת"ל (ויקרא ז, כג) כל חלב מדרבנן וקרא אסמכתא בעלמא
Rabbi Yoḥanan raised an objection to the opinion of Reish Lakish from what was taught in a baraita with regard to the prohibition of forbidden fat: I have derived only that anything that is included in the punishment of karet is included in the prohibition. However, one might have thought that there is no prohibition to eat fat of a koy, or a half-measure of forbidden fat, since there is no punishment for those. Therefore, the verse states: “All fat” (Leviticus 7:23), indicating that there is a prohibition to eat any kind of fat, including fat of uncertain status and a half-measure of fat. Therefore, a half-measure of fat is prohibited by Torah law. Reish Lakish rejects this argument: This prohibition is rabbinic, and the verse brought as a proof is a mere support. It cannot be claimed that there is such a prohibition by Torah law.
הכי נמי מסתברא דאי סלקא דעתך דאורייתא כוי ספיקא הוא איצטריך קרא לאתויי ספיקא אי משום הא לא איריא קסברי
The Gemara comments: So too, it is reasonable to say that the baraita cites only the verse as a support and not as a source to prove the prohibition. For if it could enter your mind that this teaching constitutes a prohibition by Torah law, there is uncertainty whether a koy is a wild beast or a domestic animal. Is a verse necessary to include an uncertainty? There is no doubt before God and therefore no purpose in writing a case of doubt in the Torah. Consequently, the baraita cites the verse only as a support. The Gemara answers: If that is the reason, there is no conclusive argument. The Sages of the baraita might have thought