R' Hershel Shachter on halacha changing and Daas Torah II

The main focus of the shiur on halacha changing was that if the facts change the halacha changes in response. As RHS put it, sif alef used to apply, now that the facts changed sif bet applies. He brought numerous examples (the din of b'chor, yoledes l'shiva, washing the baby with warm water after the mila, etc.) The facts may change for a number of reasons, either nature changed, or science has provided us with new/different facts. When he talked about blood tests he explained the gemara that the blood comes from the mother as an aggadic statement (which we don't really understand) having no halachic significance. He seemed to take the approach of reinterpreting the problematic gemaras. The gemara with the fish with worms growing inside is reinterpreted to mean that the fish swallowed microscopic eggs. Chazal did not know this, they thought that the worms grew inside the fish, the fact that we now know that the fish are swallowing microscopic eggs doesn't change the din.