Midrash su Levitico 21:2

4) (Vayikra 21:2) ("Only for his flesh that is near to him, his mother and his father, his son and his daughter and his brother.") "Only for his flesh that is near to him": "flesh" is his wife, viz. (Vayikra 18:12) "She is the flesh (i.e., the wife) of your father." "that is ner": to exclude one that was betrothed to him. "to him": to exclude one that he divorced. Let it be written (only) "to his mother." Why need "to his father" be written? (i.e., it can be derived a fortiori), viz.: If he makes himself tamei to his mother, who becomes a challalah (by relations with one who is unfit), how much more so should he make himself tamei to his father, who does not make himself a challal thereby! — If so, I would say: Just as his mother is definitely known (to be his mother), so his father must be definitely known. Whence would I know (that he makes himself tamei for him even when he is known as) his father by common acceptance? It must, therefore, be written "to his father."

5) Or, if it were written "to his father" and not "to his mother," I would say: If he makes himself tamei for his father, who is such only by common acceptance, how much more so does he make himself tamei for her, (who is definitely known to be his mother!) — If so, I would say: Just as his father does not make himself a challal (see above), (then he may make himself tamei) only for his mother who has not become a challalah. Whence would I know that he does so even if she has become a challalah? It must, therefore, be written "to his mother."

6) Let it be written (only) "to his son and to his daughter." Why need "to his mother and to his father" be written? (i.e., it can be derived a fortiori), viz. If he makes himself tamei to his son and to his daughter, whom he is not commanded to honor, how much more so should he make himself tamei to his mother and father, whom he is commanded to honor! — If so, I would say (that the verse speaks also of) his aborted son or daughter. It is, therefore, written "to his mother and to his father." Just as his mother and his father were alive, so, his son or daughter (must have been) alive — to exclude his aborted son or daughter, who were not alive.