Genesis 16:2

(Chapter 16, Verse 2) Behold, the Lord has made me barren, so go in to my maidservant; perhaps I shall obtain children by her. Take careful note that the procreation of children is written in Hebrew as building. For it is read there: Go in to my maidservant, that I may perhaps build through her. And see, lest perhaps this is what is said in Exodus: God blessed the midwives, and they built houses for themselves (Exodus 1:20).

The apostle saw in these women the type of the two covenants, in accordance with the rule of allegory, but since what the text narrates actually took place, the literal sense also deserves consideration. The saints entered the married life not to pursue pleasure but for the sake of children. There is in fact a tradition that says they would go with their wives only when the time was suitable for conception. They would not go with them during the lactation period, when they were nursing their young, or when they were with child, because they regarded neither of these times as suitable for coming together.…When Sarah, therefore, who was wise and holy, had observed for a long time that in spite of coming together with her husband she was not conceiving, she abstained from conjugal relations, and since she knew that it was in the order of things that he should have children, she gave him her slave girl as a concubine. This shows the moderation (sophrosyne) and the absence of jealousy of Sarah and the passionlessness (apatheia) of Abraham, who chose this solution at his wife's instigation and not on his own initiative and who yielded to her request only in order to give birth to children. The literal sense too, then, is useful according to the considerations offered above.

As for the anagogical [mystical] teaching, one could explain the text by recalling that Paul allegorically transposed the two women into the two covenants. Philo also used allegory here but giving the text another application: He understood Sarah to represent perfect virtue and philosophy, because she was a free woman and wife, of noble birth and living with her husband in lawful union. Now virtue lives with the wise man in lawful union so that he can give birth from her to a divine progeny: "Wisdom," in fact, "begets a man of discernment." In Scripture the devout and holy man is addressed with the words "your wife is like a fruitful vine.… Your children are like olive shoots around the table. So shall the man be blessed who fears the Lord."Sarah then is allegorically transposed into perfect and spiritual virtue. Hagar, the Egyptian slave, symbolizes, according to Philo, the preliminary exercises (progymnasmata), and, in Paul, "the shadow" [of good things to come]. It is not possible, in fact, to understand anything of the spiritual or elevated ideas without the shadow that is the letter or without a preliminary study of the introductory sciences, for one must first bear children from inferior unions. In the era of the shadow, they offered actual animal sacrifices, they celebrated Passover in an external and tangible way, they received physical circumcision, and all of this was preparing them gradually so that eventually they could "offer to God a sacrifice of praise," which pertains to the free woman. As the zeal of the wise impels them to go on to the higher realities in due order, virtue impels them, by divine intention, to make use first of the introductory sciences and to have children from them. Since it is impossible, in fact, for one who has just recently approached virtue so successfully to attain perfection as to have children through her too, virtue counsels such a one to subject himself first to the preparatory disciplines so that by this path he might perfectly grasp her, if he is able.