Why is the parsha of parah adumah considered such a mysterious chok when the Rishonim give reasons for it: it is the mother cow that comes to clean up the mess of the cheit ha’eigel? Chasam Sofer explains that parhas parah was one of the parshiyos given to Bnei Yisrael at Marah, right after yetziyas Mitzrayim, before the cheit ha’eigel occurred. How can G-d give a means of kaparah for a sin yet to occur without predetermining that such a sin will occur? This, writes the Chasam Sofer, was Aharon’s complaint to Moshe in response to the eigel: “Atah yadata es ha’am ki b’ra hu” – You knew this was going to happen, because there already was a parshas parah. The mystery of parah adumah is the great mystery of how yediya and bechira coincide.
It was the sin of Adam of eating from the eitz hada'as which brought death into the world. Therefore, explains the Sefas Emes, for the parah adumah to serve as a metaheir for that tumah of death it must remain an unfathomable mystery, something that our da'as cannot comprehend. The correction for the pollution of the eitz hada'as must come from a higher source than da'as itself.