Halakhah su Deuteronomio 21:17

The general rule is that a person has standing, even when contradicted by trustworthy witnesses, to declare that certain matters are prohibited to him. Thus a person may not eat a foodstuff he has declared to be non-kosher even though others are fully entitled to rely upon the contradicting testimony of two witnesses. The resultant principle is known as "Shavya anafsheih ḥatikha de-issura—He has established upon himself a prohibited piece." Similarly, a person has credibility to declare himself a mamzer and consequently forbidden to marry a Jewess of legitimate birth. Although such credibility is generally limited to oneself, nevertheless, as recorded by Rambam, Hilkhot Issurei Bi'ah 15:15-16, a father is granted specific authority to declare his son to be a mamzer, most particularly when he testifies directly with regard to his son's status. That authority is derived from the biblical verse "yakir— he shall recognize" (Deuteronomy 21:17). If the father testifies that he himself is a mamzer and, by implication, that status is shared by his son as well, some authorities maintain that the father's testimony creates only a doubtful status with regard to the son.

And Rambam (Sefer HaMitzvot LaRambam, Mitzvot Ase 248), may his memory be blessed, wrote that included in these laws of inheritance without a doubt, is that the first-born receives double - as this is included in the commandment of inheritance. And the explanation of double is two portions, meaning that if there are two brothers, we [divide] the money into three parts and the first-born takes two. And if there are three, we [divide] it into four parts and he takes two, and so forth forever. But the first-born only takes double in properties that are held by the father (Mishneh Torah, Laws of Inheritances 3:1), meaning to say that came into the domain of his father in his lifetime, but not that which is fitting to come to his father, as it is stated (Deuteronomy 21:17), "of all that is found to him." How is this? [If] one of the individuals from whom the father could inherit dies after the father, the first-born does not take double from that inheritance. And so [too,] if the father had a debt that was owed to him (Bava Batra 125b) - even [if it was recorded] in a contract - or if he has a merchant ship [raising revenue] in the sea, the first-born does not have double of this; as this is not 'found to the father.' And so [too,] for this reason, the first-born does not take double of the appreciation of the properties after the death of his father, such as a field yielding grain, or trees bearing their fruit, as these were not extant to his father in his days. If, however, a small tree grew larger not due to an expense - but rather grew on its own - after his father's death before the division [of the property], the first-born does take double of this; as this is considered extant to his father, as it is unchanged in its form.

And the father is believed [when] he says, "This is my son, my first-born" (Kiddushin 74a), or to say, "This is my son" or "This is not my son." As it is written (Deuteronomy 21:17), "But the first-born etc. shall he recognize" - and they, may their memory be blessed, expounded (Bava Batra 127b), "shall make him recognized by others." And even if someone was presumed to be Reuven's son, if Reuven says that he is not his son, he is believed, and he does not bequeath to him. And Rambam (Mishneh Torah, Laws of Inheritances 4:2), may his memory be blessed, wrote, "It seems to me that even if the son had sons, [such that] even though [the father] is not believed about him to say he is not his son, for the purposes of family relationships, and he is not presumed to be a mamzer on his word, he is believed for the purposes of inheritance and he will not inherit him.