Der Alter seems pretty convinced that if Chazal or Rishonim didn't say it, then it can't possibly be 'Authentic Torah True Judaism'. Hence my peshat in Breishis 1-11 cannot possibly be correct. By the same logic no peshat which states the world is older than 5765 years, or that Adam & Eve were not real, or that there was no global flood can possibly be Torah True.
Now, I have the greatest respect for Chazal and Rishonim, and for Der Alter too, especially in this month of Ellul, prime Mussar season. Mussar is supremely important and I think its a crying shame that we all don't learn more of it. R Yisroel Salanter is one of my heroes for re-emphasizing the ethical, moral and spiritual aspects of Judaism. So far be it from me to bash the heilige Chazal, Rishonim or Mussar Greats.
But I must respond with this: WHAT A LOAD OF NONSENSE.
First of all, there is plenty that Chazal and Rishonim said which is not part of Judaism (think bogus medicine and spiritual moons). Der Alter claims these were never part of Judaism itself, just external items which were deemed 'compatible with Judaism'. OK. Then think about all the new stuff which was invented after the Rishonim (think Ari's Kabalah). Der Alter claims these were all based on previous authentic strands of kabalah. Whatever.
The fact of the matter is this: It doesn't matter one whit what Chazal or Rishonim thought was peshat in Breishis, since CHAZAL AND THE RISHONIM HAD ABSOLUTELY NO CLUE HOW OLD THE UNIVERSE WAS OR HOW HUMANITY EVOLVED OR WHETHER A GLOBAL FLOOD HAD HAPPENED. Sorry, but it's true. And this is no disrespect to them AT ALL. THEY COULDN'T POSSIBLY HAVE KNOWN, SINCE NO ONE ON THE ENTIRE PLANET KNEW THESE THINGS before about the 18th century. Isn't this BLINDINGLY OBVIOUS TO ANYONE WITH THE REMOTEST DEGREE OF SECHEL?
This being the case, ANY UNDERSTANDING WHICH CHAZAL AND RISHONIM HAD IN BREISHIS IS ALMOST ENTIRELY IRRELEVANT. I say 'almost' because any moral, ethical or spiritual lessons they learned are 100% relevant. But the fact that Chazal and the Rishonim assumed the world was only a few thousand years old, or that Adam & Eve were real, or that the flood was global is irrelevant. They assumed it was literal because they had no reason not to assume that. But we HAVE EVERY REASON IN THE WORLD TO KNOW IT CAN'T POSSIBLY BE LITERAL.
So, we are left with a choice. Either kvetch like crazy and come out with some bastardized peshat in Breishis which barely conforms to reality and turns Breishis stories into a joke, or recognize that it's Mythology and learn the correct lessons from it. Again, MYTHOLOGY DOESN'T MEAN FAIRY TALES. MYTHOLOGY MEANS IMPORTANT INFORMATION ENCODED INTO SIMPLE STORIES.
Go read some books on mythology, especially this one. There may even be kernels of true events encoded in the Breishis stories. But you won't get at any of this with kvetchy peshat and bogus science. A far better approach would be to read up on ancient Sumerian Mythology and compare and contrast. Read Cassuto: From Adam to Noah, or Sarna: Understanding Genesis. Or see the notes at the end of Breishis in the standard Soncino Chumash.
The funny thing is that this MYTHOLOGY/MOSHOLOGY peshat is standard amongst educated MO Jews and even some LW UO Jews (don't make me name any names!). It's only sheltered Chareidim who are shocked to hear of it. For goodness sake guys, it's been in the Soncino Chumash since 1936! And Rabbi Hertz was the Chief (Orthodox) Rabbi of the British Empire!
I love Judaism and I love Mussar, but I cry for both.