moshe heller asked:
hamanach kelim tachas hazinor:according to the first pshat of the gemara that the reason it will pasul the mikveh because his first machshava was batul but what is the gezera ,there is no gezera it is just a machlokes about whether his first machshova was batul and we paskin like bes shamei??!!
moshe heller, La, CA
The Kollel replies:
The Sfas Emes (end of 16b) asks your question, and answers that it must mean that Beis Shamai was Gozer that his first thought is not Batel.
All the best,
Yaakov Montrose