The Gemara says that if a male animal mounted a Parah Aduma for mating, it is unfit for use in the purification ritual. עלה עליה זכר - פסולה!
Asked Tosfos:
וא"ת ועלה עליה זכר אמאי פסולה הא ודאי לא ניחא ליה להפסיל פרה שדמיה יקרים בשביל דבר מועט
When a male mated with it, why is it Pasul? Surely he is not pleased that his Parah, which is very valuable [when it is Kosher], be disqualified for a small matter!
וי"ל דאם היתה כשירה הוה ניחא ליה ולכך אין להכשיר
Answer: If it would be Kosher, he would be pleased. Therefore, we are not Machshir it.
BUT WAIT!!! If we are not מכשיר the פרה if the male mounted it, then he is not pleased - לא ניחא ליה - and therefore it should be kosher! Ahhh - it remains kosher? So he is pleased with it - ניחא ליה!! Ahhh - ניחא ליה? It is פסול! Pasul? It is not ניחא ליה! Not ניחא ליה? Kosher!! Kosher? He is ניחא ליה! Ahhhh - ניחא ליה? Pasul....
So really this is a endless cycle. So how can Tosfos decide that "ניחא ליה" and it is פסול? Why be machmir? Why be such a Brisker??
Explained the Ponovitcher Rov - Tosfos isn't being מכריע to one side. It is a cycle and each side brings the other. The only thing is that in order to be מכשיר, we need a DEFINITIVE "לא ניחא ליה" and THAT - we don't got [sorry for the English - spent a lot of time playing basketball with chevre who would speak that way].
NOTE: Tosfos doesn't say that it is פסול - just that we can't be מכשיר.
Listen here for a GEVALLLLLDIIKKKEEEEEE Munkatcher proof [from a Torah journal printed in 1922 - the year that Readers Digest was created and the NY Giants ("ענקים מנוי יורק") didn't win the Super Bowl - because there was no Super Bowl back then - but the World Series. They beat the Yankees. The Munkatcher was NOT a Yankees fan so he didn't care] for a different yesod - when we have a גלגל החוזר - endless cycle when היתר brings איסור which brings היתר etc. we are מחמיר when it comes to דיני דאורייתא. [What a stroke of mazel that I found a shiur that talks about this topic!! 😊😊].
