M.L. West seems pretty confidant that ereshi is akin to Ger. raissan which is PIE ergh, but shiply groups ergh with *rei and *res and river is lumped with rave and rage. If I'm not mistaken Av. Urvan "soul" and NPer. Ravan shenasi "psychology" and Rohani "spiritual leader" are indirectly relate to Av. Ereshi.My point is simply that ərəši- occurs exactly once in the Avesta, in a very obscure passage. Nobody really knows what it means. In this circumstance it is not possible to propose a likely etymology.
My colleague M L West is a very distinguished classicist. Many years ago he wrote a book about Indian parallels to the Greek philosophers. This was severely criticised at the time. It was pointed out in particular that he would never have tolerated anyone writing about Greek authors if that person did not know Greek, and asking how West could venture to write about Indian texts without knowing any Sanskrit. West took the criticism to heart and actually learned Sanskrit, and Avestan too. Recently he has published his translation of the Gathas. This too has been rather fiercely criticised by specialists in Iranian studies. West’s strong point is that he actually tries to make sense of the texts rather than just translating words at random (as some others have done). But sometimes he bends the meaning of the words just a bit too much. The Gathas do remain an extremely obscure work.