Deuteronomy 25:5

In the third book, then, when I was solving the question of how it was possible for Joseph to have two fathers, I indeed said that "he was begotten by one and adopted by the other." But I should have mentioned too the kind of adoption, for what I said sounds as if another living father had adopted him. The law, however, also adopted the children of the deceased by ordering that "a brother marry the wife" of his childless, deceased brother and "raise up seed" by the same woman "for his deceased brother." In this way the explanation of this matter of the two fathers of one man is indeed made clearer.

From the law on Levirate. "If two brothers live together, and one of them die without children; the wife of the dead will not be to another who is not near to him; her husband's brother will go to her, take her for a wife, and live with her. And the child who will be born will be admitted in the name of the deceased, and his name will not be erased from Israel. In wanting the widow to be married by her husband's brother, the Law seems to have had no other purpose than to raise a posterity to the one who died without children. Now, these words: "The child shall be admitted in the name of the deceased, and his name," that of the deceased, "shall not be removed from Israel," appear to mean that by taking the name borne by the deceased, the child will in a way form his posterity. Also, in the question raised by the Gospel, about the two fathers of St. Joseph, one of whom, quoted by St. Matthew, begot Joseph, and whose other, named by St. Luke, had Joseph as son, we have admitted adoption as the most plausible solution of this apparent contradiction (Accord des Evang. liv II, c. 3; Matt.I, 16; Luc, III, 23); since St. Joseph did not name either of them. But perhaps these words: "He will be admitted" in the name of the deceased, "do not they mean that he will take his name, but that he will be established by this heir, that is to say, as a son, not of him who begat him, but of the dead, to whom were raised children. Indeed, these expressions: "And his name will not be erased" from Israel, "may mean, not that the child will be required to bear the name of the deceased, but that the latter will seem not to be dead without posterity and that his name, that is to say, his memory will thus be saved from oblivion. Without a doubt,. if he had had a son himself, he would not have needed to give him his name, to render him imperishable in Israel; but by this alone that he would not have died without posterity, his name would not have perished: now, which was not in his power, the Law commands him to his brother, ordering him to marry the widow of the deceased. For the rest, in the absence of the brother, the nearest relative married the woman of the dead Israelite without children, in order to raise him a posterity: this is how Booz married Ruth, in order to raise children to a close relative, she had been the wife, without having children from him; the son who was born…