Bava Mezia 93b. There was a shepherd who was shepherding his sheep and one fell into the river. Raba said the shepherd is not liable for what could he have done? That is he considered this a case of a a unavoidable (big) accident that a paid guard does not pay for. Abayee asked on this from a braita a shepherd was watching his sheep and went into the city for a  short break and a lion came and took one of the sheep. If he could have saved the sheep if he had been there he is  liable. Abayee suggested that the break was a normal kind and yet he is still liable and so we see this is considered a case of an  avoidable accident [small accident]and so the shepherd is liable. Raba answered, no. rather it is a case of taking an unusual break and so it is considered an avoidable accident but for an unavoidable accident  he would not be liable. abaye asked if the case is he took an unusual break then it it is a case of the beginning  in transgression and the end in accident is liable. Tosphot learns from here that a minimal amount of guarding is not considered the beginning  in transgression and the end in accident.

The Ramban brings the gemara in Bava Kama 45a to show that this same principle is accepted universally, not just for Abayee. There the teaching is that four types of guards take the place of the owner. If they are guarding an dangerous ox (that had been seen to kill on three other occasions) that got loose and killed someone. The ox is killed, and the guards except the unpaid guard pay kofer a fine and pay for the loss of the ox to the owner. So we see by the fact that the unpaid guard is not liable that the minimal amount of guarding is not considered transgression (for all guards). My question here is that perhaps a minimal amount of guarding is not considered transgression for an unpaid guard, but perhaps it is considered transgression for a paid guard since he is obligated in a higher standard of guarding? [I noticed later that the Ramban, in fact, only meant his argument to apply to the unpaid guard.]

I might mention that Rav Shach and Rav Meltzer had an argument about this Ramban. Rav Meltzer claimed the argument of the Ramban only worked if the obligation of the guard is towards the owner of the animal, while Rav Shach showed that the Ramban's argument works even if the guard is obligated toone whom gets injured by an animal he was guarding.

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בבא מציעא צ''ג ע''ב. There was a shepherd who was shepherding his sheep, and one fell into the river. רבה said the shepherd is not liable for what could he have done? That is he considered this a case of a a unavoidable (big) accident that a paid guard does not pay for. אביי asked on this from a ברייתא a shepherd was watching his sheep and went into the city for a  short break and a lion came and took one of the sheep. If he could have saved the sheep if he had been there he is  liable. אביי suggested that the break was a normal kind, and yet he is still liable; and so we see this is considered a case of an  avoidable accident [small accident] and so the shepherd is liable. רבה answered, no. Rather it is a case of taking an unusual break and so it is considered an avoidable accident, but for an unavoidable accident  he would not be liable. אביי asked if the case is he took an unusual break, then it it is a case of "the beginning  in פשיעה and the end in אונס is liable." תוספות learns from here that a minimal amount of guarding is not considered "the beginning  in פשיעה and the end in accident."

The רמב''ן brings the גמרא in בבא קמא מ''ה ע''א to show that this same principle is accepted universally, not just for אביי. There the teaching is that four types of guards take the place of the owner. If they are guarding an dangerous ox (that had been seen to kill on three other occasions) that got loose and killed someone. The ox is killed, and the guards except the שומר חינם pay כופר a fine and pay for the loss of the ox to the owner. So we see by the fact that the שומר חינם is not liable that the minimal amount of guarding is not considered transgression. My question here is that perhaps a minimal amount of guarding is not considered transgression for an unpaid guard, but perhaps it is considered transgression for a paid guard since he is obligated in a higher standard of guarding?

בבא מציעא צ''ג ע''ב. היה רועה צאן ואחד נפל לנהר. רבה אמר שהרועה אינו אחראי. מה הוא היה יכול לעשות? כלומר הוא ראה בזה מקרה של תאונה בלתי נמנעת (גדולה) ששומר בתשלום לא משלם עליה. אביי שאל על כך מברייתא רועה צאן שומר כבשיו ונכנס לעיר להפסקה קצרה, ובא אריה ולקח את אחת הכבשים. אם הוא היה יכול להציל את הכבשה לו היה שם הוא אחראי. אביי הציע שההפסקה היא רגילה, ובכל זאת הוא עדיין אחראי; ולכן אנו רואים שזה נחשב למקרה של תאונה נמנעת [תאונה קטנה] ולכן הרועה אחראי. רבה ענה לא. אלא מדובר בהפסקה חריגה, ולכן היא נחשבת לתאונה שאפשר להיות נמנעת, אך בגין תאונה בלתי נמנעת הוא לא יהיה אחראי. אביי שאל אם במקרה הוא לקח הפסקה חריגה, אז זה מקרה של "ההתחלה בפשיעה והסוף באונס שהוא אחראי". תוספות לומדת מכאן שכמות מינימלית של שמירה אינה נחשבת "התחלה בפשיעה וסוף בתאונה.

הרמב''ן מביא את הגמרא בבבא קמא מ''ה ע''א להראות שאותו עיקרון מקובל בכל העולם, לא רק על אביי. שם ההוראה היא שארבעה סוגי שומרים תופסים את מקומו של הבעלים. אם הם שומרים על שור מסוכן (שראו אותו הורג בשלוש הזדמנויות אחרות) שהשתחרר והרג מישהו. השור נהרג, והשומרים חוץ מהשומר חינם משלמים כופר (קנס) ומשלמים על אובדן השור לבעלים. אז אנו רואים בעובדה שהשומר חינם אינו אחראי שכמות השמירה המינימלית אינה נחשבת לעבירה. שאלתי כאן היא שאולי כמות שמירה מינימלית אינה נחשבת עבירה לשומר ללא שכר, אבל אולי היא נחשבת עבירה לשומר בתשלום שכן הוא חייב ברמת שמירה גבוהה יותר?