You learn פרק אלו מציאות and you see time and again the concept of יאוש and everything is cool until you wake up in the middle of the night in a cold sweat and ask yourself "WHERE DO WE KNOW THAT THERE IS SUCH A CONCEPT CALLED יאוש?" So I despaired of finding my car, that allows you to keep it??? What is the source?
You scramble around looking for a source [your sweat is getting colder] and you find a Tosfos [כ"ז. ד"ה מה] that takes a stab at it and then quotes a Yerushalmi [why does it need an independent source if it has a Yerushalmi?]. Great!! Ahhhh - there is also a Rashi all the way out there in Bava Kamma [ס"ו] who also talks about it.
But how come the gemara [Bavli] never mentions one?? ALL OVER THE PLACE the gemara searches for sources of laws and here it is just taken as a given. The Yerushalmi found it necessary to locate a source so why does the Bavli remain silent? Especially given the fact that it is a difficult concept to digest. The gemara doesn't offer a svara either.
Anyway - please think about it...