בבא בתרא קמו:
"מאן תנא דאזלינן בתר אומדנא"
The Gemara asks, who is the תנא of our Mishnah who holds when determining the mindset of a נותן מתנה we follow the אומדנא and even if not clearly articulated, we assume that was his intention? ר' נחמן says it is ר' שמעון בן מנסיא from the following Braisa: If one’s son was overseas & upon hearing that he died, the father bequeaths his entire estate to someone else. If afterwards, his son is discovered to be alive, ר' שמעון בן מנסיא says the מתנה is void, because had the father known his son was alive, he never would have given it away to someone else. ר' ששת says it’s a different תנא, not ר' שמעון בן מנסיא, because his psak in this case does not prove he holds of a standard אומדנא since it’s very obvious what the father was thinking and this would be considered a "אומדנא דמוכח" not a standard אומדנא.
The רמב"ם [זכיה פ"ו, ה"א] says, "לעולם אומדין דעת הנותן" and brings our case of ר' שמעון בן מנסיא to demonstrate that we follow דברים המוכיחין even if not stated specifically. The [שו"ת קכט] מהרי"ק points out that often in ש"ס we follow the אומדנא and sometimes we don’t. For example; we learned [ב"ב צג.] if a שור was gored to death in the field and standing next to it is a שור המועד, the רבנן say we do not follow the אומדנא and say this שור המועד is the killer. In the next case, ר' אחא says if a camel is found dead and a wild camel is discovered next to it (גמל האוחר בין הגמלים), we do say this must be the killer camel. The מהרי"ק brings the רמב"ם [נזקי ממון פ"ח, הל' יד] also the רי"ף וסמ"ג who all pasken in both cases like the רבנן that we do not follow אומדנא. Yet in our case ofthe father’s מתנה we take the אומדנא into consideration. What is the difference? The מהרי"ק explains a כלל If the מעשה is clear cut in front of us but we’re just not sure of his intentions, then we assume the אומדנא. But if the actual circumstances of theמעשה are in doubt like the שור המועד and גמל where there aren’t witnesses, then we don’t apply אומדנא.
The [אהע"ז ח"א, ס' קיט,אות קד] אבני נזר quotes the רמב"ם [סנהדרין פ"כ,א], if witnesses observe a man chasing someone into a חורבה and they follow and see one of them holding a bloody sword, standing over the other one who is dying on the ground, since they didn’t witness the actual murder, we may not convict based on these עדים because the Torah says "ונקי וצדיק אל תהרוג" as long as there is a צד לנקותו we may not kill him. The אבני נזר says it seems from the reason the רמב"ם gives, אומדנא isn’t usable because it’s a case of דיני נפשות but if this אומדנא was available in a case of דיני ממנות we would use it. Yet, we see that in the case of גמל we pasken not to use אומדנא? He explains there are different levels of אומדנא. If, in דיני ממונות there would be a clear [דמוכח] אומדנא like the case in the Rambam, we would follow it. But since the case of גמל is not as clear, thus we don’t use אומדנא.