בבא בתרא קלד.
"האומר זה בני נאמן"
If a person says, “This is my son,” he is believed. The [ד"ה ליורש] רשב"ם explains that although earlier we learned from the word "יכיר", that a father is נאמן to say זה בני בכור, that is only when we already know this is his son, we just don’t know if he is the בכור. Our Mishnah is referring to where we have no prior knowledge at all if this is even his son. The רשב"ם learns that the אב is only נאמן regarding entitling him to פי שנים on the assets he owns now. This is because the father has a מיגו and can gift this property to him now if he wants. Consequently, since this נאמנות is working only because of a מיגו not because of the דין of יכיר, the father is only נאמן to allow his son to take a double portion on his current assets. The [פ"ח,ס' לט] רא"ש says this פשט is a פליאה גדולה and disagrees. He holds that יכיר gives the father נאמנות even on a person that was never הוחזק to even be a son. Therefore, since the נאמנות is not working מכח מיגו, the אב is believed and he receives פי שנים on all נכסים. The [נחלות פ"ב,ה"יד] רמב"ם holds like the רא"ש, that one is believed to say זה בני, even on someone who is a stranger to us.This is מדין יכיר and not מדין מיגו and applies to all נכסים. The רמב"ם [נחלות פ"ד הל' ב'] also says that יכיר gives the father נאמנות to say בני זה ממזר. [קכז:] ר' יהודה says just like the אב is נאמן to say זה בני, so too, he is נאמן to say זה בן גרושה וזה בן חליצה. Tosfos [ד"ה כך] asks, just because יכיר teaches us נאמנות regarding ירושה, how can we extend this נאמנות to פסלות and believe the father לפסול את בנו? Tosfos answers since יכיר allows the father to identify a younger son as the בכור, thus in essence saying the older ones are ממזרים, so too, it gives him the נאמנות to say זה בן גרושה. The [קכז:] מאירי says that when the father says זה בן גרושה, he must say he married the גרושה בשוגג, otherwise he would not be believed because "אין אדם משים עצמו רשע". The מאירי [סנהדרין ל:] also explains that if a son brings עדים that testify that they heard his father say he is the בכור, they are believed and he receives פי שנים. The מאירי explains that this is not a חסרון of עד מפי עד, because the father’s נאמנות is like a הודאת בעל דין and it’s as if the עדים heard ראובן say he owes שמעון money which they may testify to and not be considered עד מפי עד. [חשוקי חמד] ר' זילברשטיין brings a story where a father said on his son who was a מוחזק בכור, “I am מסופק if he is even my son.” The דיינים were discussing if this son could still collect פי שנים, or do the words of the father create a ספק and contradict the current חזקה? ר' זילברשטיין quotes the [אבהע"ז ס' ד', כט] שו"ע that a father is נאמן to say אינו בני. He then brings the תשב"ץ who says this נאמנות is valid only when the father says ודאי ממזר, but not if he is מסופק. He also brings the [אבהע"ז ס' ד' לז'] פתחי תשובה who says בשם שו"ת רע"א that a father is נאמן to say זה אינו בני only if he is ברור, but if he is in doubt this is not called a הכרה. He concludes, that we see clearly from this that a הכרה בספק is not considered a הכרה.