באיסור הבא מאליו באיסור הבא על ידי עצמו לא אמרינן
it is with regard to a prohibition that occurs on its own, like the prohibition against eating on Yom Kippur, which is more inclusive than the prohibition against eating non-kosher food and therefore takes effect. But with regard to a prohibition that occurs by the act of a person himself, i.e., an oath or a vow, we do not say that because it is more inclusive it can take effect also with regard to items that are already forbidden by Torah law.
בשלמא לריש לקיש משום הכי קא פטר רבי שמעון דתניא רבי שמעון אומר כל שהוא למכות ולא אמרו כזית אלא לקרבן אלא לרבי יוחנן מאי טעמא דרבי שמעון דפטר
The Gemara asks: Granted, according to Reish Lakish, who understands the mishna to be dealing with the case of one who took an oath about less than a full measure, it is due to this reason that Rabbi Shimon deems exempt one who takes an oath prohibiting himself from eating non-kosher food, as it is taught in a baraita: Rabbi Shimon says: Any amount is sufficient to render one liable to receive lashes, and the Sages stated the measure of an olive-bulk to determine only liability to bring an offering. According to Reish Lakish, Rabbi Shimon holds that one is already under oath from Mount Sinai even with regard to less than a full measure, and for that reason the oath does not take effect. But according to Rabbi Yoḥanan, who understands the mishna to be referring to an oath that includes erstwhile permitted items, what is the reason that Rabbi Shimon deems one exempt from bringing an offering for breaking his oath not to eat non-kosher food?
מידי הוא טעמא אלא משום איסור כולל רבי שמעון לטעמיה דלית ליה איסור כולל דתניא רבי שמעון אומר האוכל נבילה ביוה"כ פטור
The Gemara answers: Isn’t the reason that the first tanna holds that the individual is liable for violating his oath only due to the fact that the oath generates a more inclusive prohibition? In this matter Rabbi Shimon conforms to his standard line of reasoning in that he does not hold that a more inclusive prohibition takes effect where a preexisting prohibition is in place. This is in accordance with that which is taught in a baraita: Rabbi Shimon says: One who eats non-kosher meat on Yom Kippur is exempt from bringing an offering for eating on Yom Kippur, despite the fact that the prohibition against eating on Yom Kippur is a more inclusive prohibition than that of eating non-kosher meat, as on that day one may not eat anything.
בשלמא לריש לקיש משכחת לה בלאו והן אלא לר' יוחנן בשלמא לאו משכחת לה אלא הן היכי משכחת לה
From the verse: “Or if any one take an oath clearly with his lips to do evil, or to do good” (Leviticus 5:4), the Sages derived that one is liable to bring an offering for an oath on an utterance only when the oath is such that it could be inverted from the positive to the negative or vice versa. For example, one is liable for violating an oath to eat because one can also take an oath not to eat. The Gemara asks: Granted, according to the opinion of Reish Lakish, you find a case where the oath can be negative or positive. Therefore, one is liable according to the Rabbis when he takes an oath that he will eat any amount, since he could also take an oath that he will not eat any amount. But according to the opinion of Rabbi Yoḥanan, granted, that you can find a case of a negative oath, as the oath not to eat non-kosher animals takes effect when it includes other erstwhile permitted items. But how can you find a positive version of this oath? An oath to eat non-kosher animals cannot take effect, as eating non-kosher animals is prohibited by Torah law.
אלא כדרבא דאמר רבא שבועה שלא אוכל ואכל עפר פטור
Rather, do not distinguish between the former and the latter clauses of the mishna based on whether he specifies what he is eating. In both cases he takes an oath not to eat, without specifying. In the case where he eats something inedible, he is exempt, in accordance with that which Rava says, as Rava says that if one said: On my oath I will not eat, and he ate dirt, he is exempt, since eating an inedible substance is not considered to be eating. Eating non-kosher meat is considered to be eating; for that reason, the latter clause of the mishna states that one is liable for doing so if he took an oath not to eat. The oath takes effect with regard to the non-kosher items because, as Rabbi Yoḥanan noted, it includes items that would otherwise be permitted.
אמר רב מרי אף אנן נמי תנינא קונם אשתי נהנית לי אם אכלתי היום ואכל נבילות וטריפות שקצים ורמשים הרי אשתו אסורה לו
Rav Mari said: We learn in the mishna (22b) as well that eating non-kosher food is considered eating, as if one said: It is konam for my wife to derive benefit from me if I ate today, and he had eaten carcasses or tereifot, repugnant creatures or creeping animals, his wife is prohibited from deriving benefit from him.
הכי השתא התם כיון דמעיקרא אכל והדר אשתבע ליה
The Gemara asks: How can these cases be compared? There, since he initially ate the forbidden item and only then took an oath saying that he did not eat,