Yevamos 39b - Abba Shaul: Wrong Intentions Make the Child a Mamzer

Abba Shaul says that one who has improper intentions when performing the mitzvah of yibum (i.e. he is motivated by physical desires rather than the intent of performing the mitzvah) is "as if he had relations with an ervah" and there is a concern of the child being a mamzer. The opinion of Abba Shaul is very difficult:

1. From the phrase it sounds as if he is not convinced that the intentions actually make a difference?

2. R' Elchonon (kovetz hearos 36:1) points out that abba shaul and the rabbanan seem to be arguing about 2 things. First, acc. to rabbonon he can acquire her even if he has the wrong intentions - abba shaul argues and says that he would not be koneh her. Second, the rabbonon would say that even if he is not koneh her, there is also no issur of eishes ach, similar to a miuberes acc. to reish lakish (when she later miscarries), although he did not perform yibum and was not koneh her, there is no issur eishes ach - abba shaul disagrees and says there is an issur eishes ach.

3. The Poras Yosef points out that from the fact that the mishnah in bechoros is based on abba shaul, it implies that even though originally they had pure intent for mitzvah, and later they did not have pure intent for mitzvah but also had intent for beauty, abba shaul's ruling applies that there is a concern of issur eishes ach. This seems to contradict what we normally assume by mitzvos. Even acc. to the opinion that mitzvos require kavana, one can have an ulterior motive as well and that would not detract from the kavana of the mitzvah. Here by Yibum, abba shaul seems to require PURE kavana for mitzvah without any ulterior motive. Clearly, his requirement is not to have kavana for mitzvah, but rather NOT to have ulterior motives.