In Parashat Chayei-Sara, the Torah tells of the search for a wife for Yitzchak, and it introduces this section with the verse, "Avraham was old, advanced in years, and the Lord had blessed Avraham with everything" (24:1). The Gemara, in Masekhet Bava Batra (16b), records several views among the Tanna'im as to the deeper meaning of God's having blessed Avraham "with everything." Rabbi Meir, the first Tanna cited, explains this phrase to mean that Avraham never had a daughter, and in this sense he was "blessed with everything." Why does Rabbi Meir consider it a blessing not to have a daughter? In fact, Halakha follows the view in Masekhet Yevamot (61) that one fulfills the mitzva of procreation only once he has at least one son and one daughter. In what sense, then, was it a "blessing" for Avraham not to have begotten a daughter?
The Ramban, and, in further elaboration, Rav Shimshon Refael Hirsch, explain that in this particular instance, we can easily understand why – at least according to Rabbi Meir – the lack of daughters is to be viewed as a blessing. If Avraham had a daughter, she would inevitably be lost to idolatry. After all, marriage in the ancient world meant the wife's entry into the husband's family, which naturally entailed her integration into his entire milieu and culture. It is hardly likely that Avraham would find a family with a religious orientation suitable for his daughter. In the case of Avraham's son, of course, the girl would join Avraham's family, and it was understood that she and her children would ultimately embrace the culture of Avraham. If he had a daughter, however, she and her progeny would have undoubtedly become idolaters.
The second view recorded in the Gemara, that of Rabbi Yehuda, holds that to the contrary, the all-encompassing "blessing" mentioned in this verse refers to the fact that Avraham had a daughter. How would he respond to Rabbi Meir's argument, as understood by the Ramban, that the impossibility of finding a suitable match for a daughter renders it a blessing that Avraham did not beget a daughter? The Ramban writes, "This is the blessing 'with everything,' for he had everything that people long for, lacking nothing." Despite the problem in finding a suitable husband, Rabbi Yehuda still considers having a daughter a necessary component of a blessing "with everything," since people long to have both a son and a daughter.
Rav Hirsch explains Rabbi Yehuda's view differently, explaining that God blessed Avraham not only with a daughter, but also in the sense that she remained loyal to his teachings after marriage. Additionally, Rav Hirsch writes, through this daughter's marriage, Avraham "formed a bridge between the isolated house of Avraham and the rest of the world. Avraham was thereby granted a blessing that would have remained unobtainable in his isolation: to plant the Abrahamitic spirit in the non-Abrahamitic world." Avraham's daughter enabled him to establish a bond with the outside world, which represented the ultimate "blessing" for which he could have hoped.