ויקרא, פרק י״ח, פסוק י׳

The laws of forbidden relationships draw clear boundaries around immediate family members. Among these is the strict prohibition against engaging in intimate relations with a granddaughter, whether she is the child of a son or a daughter. The rationale provided for this restriction highlights a deep biological truth: unlike other forbidden relationships that stem from marriage or extended family ties, this is a matter of direct bloodline. Granddaughters are considered a person's very own flesh. Consequently, violating their boundaries is equivalent to exposing the person's own shame and dishonor [שד״ל, רש״ר הירש, ביאור שטיינזלץ, רלב״ג].

A close look at this restriction raises an obvious question: while the granddaughter is explicitly forbidden, the more fundamental prohibition against relations with one's own daughter is seemingly omitted. The primary approach among commentators is that there is no need to mention a daughter born from a legal wife in this context. That specific restriction is already encompassed by a later law prohibiting relations with both a woman and her daughter. A man who engages in relations with his daughter from his wife automatically violates that broader rule [שד״ל, ביאור יש״ר, בכור שור].

If daughters and granddaughters from a legal marriage are already forbidden by other laws, the purpose of this specific restriction must be understood differently. Commentators agree that it establishes boundaries regarding blood relatives born outside the framework of marriage, such as a child conceived from rape. While other laws address relationships formed through legal marriage, this rule specifically teaches that a man is forbidden to have relations with his biological granddaughter born out of wedlock. Importantly, if the mother of that out-of-wedlock child had another granddaughter from a different man, that granddaughter would be permitted to him, as the older woman is not his legal wife [רש״י, מזרחי, תורה תמימה, שפתי חכמים, גור אריה, ברכת אשר על התורה].

If this law focuses on descendants outside of marriage, how is the out-of-wedlock daughter herself forbidden, given that she is not explicitly mentioned? Common sense would dictate a simple logical deduction: if a granddaughter is forbidden, surely the daughter is as well. However, commentators emphasize a core legal principle that biblical prohibitions and punishments cannot be established based solely on human logic. Instead, the sages derived the ban on the out-of-wedlock daughter through a traditional method of comparing matching terms across different laws. By linking the phrasing here with the law regarding a woman and her daughter, they established that just as the latter applies to both a daughter and a granddaughter, the prohibition against out-of-wedlock descendants applies equally to the daughter [רש״י, רבנו בחיי, ברטנורא, חזקוני].

Building on this, another perspective notes that the fundamental reasoning—that the granddaughter is considered the man's own flesh—naturally relies on the existence of the daughter. The granddaughter is only viewed as an extension of the man because the daughter herself is intimately identified with him. As a result, the daughter is inherently and essentially included within the very fabric of this prohibition [פירושי רד״צ הופמן].