Musar su Genesi 38:8

The true reason is that the Talmud, when giving the views of the two Rabbis Yossi, stated: "One said this and the other said that." The fact that the Talmud did not identify who said what means that the two Rabbis did not disagree. Each only cited one possible reason for the different opinions. The lights allude to the souls, as we know from Proverbs 20,27: נר ה' נשמת אדם, "The soul of man is the light of the Lord." We know that nowadays most of the souls inhabiting bodies are re-incarnations of people who have lived in earlier times. The best of such kind of re-incarnation one can experience is if the soul is allotted to someone who is related to the former incarnation, as has been explained by the Rekanati in connection with the legislation of the levirate marriage. When Yehudah said to his son Onan that he should marry the widow of his brother Er (38,8), he meant that the first child to be produced from such union would become the re-incarnation of Er. Later on his intention of having Er re-incarnated became fulfilled through his own union with Tamar. When Peretz and Zerach were born by Tamar, they were the respective reincarnations of both Er and Onan (Onan also having died in the meantime). The Rekanati explains this at length. When the Talmud said that the essence of the Chanukah light is that a person should light one light for his house- read: "family"- this is an allusion to the duty of ensuring re-incarnation by means of the levirate marriage.