Halakhah su Genesi 38:9

And according to that which we mentioned, when the verse states (Genesis 38:9), "But Onan, knowing that the seed would not be his," its sense [in terms] of the simple meaning is to say that not all of the merit of the seed would be his, as his brother would take a part in it; and he was not [interested] in partial merit. And also because maybe the main merit is to the dead brother, since he is like the owner of the field and the living brother is like a sharecropper - and like the well-known matter about sharecroppers, that there are some that provide their own seed. And this is what they, may their memory be blessed, said (Yevamot 22b) that any time that his brother has any memory in the world - a son or a daughter or grandchildren from another woman, and even an illegitimate son (mamzer) or daughter - that they exempt his wife from levirate marriage; [such] that it appears from this that the matter is only to memorialize his name and to give him a portion and merit in this physical world (see Guide for the Perplexed 3:49). And behold, with my knowing that there is a major principle and a correct explanation about this commandment with the Kabbalists, I will rely upon what I wrote at the beginning of my book. And there I penned my apology, that I do not silence myself from writing my thoughts about the simple understanding of the explanations of the commandments, in order to stimulate the spirit of the children to ask questions about them to their elders and their teachers. By [their] dealing with them, perhaps, a merit will come through me in the revelation of the truth of their matters, and I will merit with them in my place.