Purification through bittul of intellect

ובטהרת פרה כתיב חוקה.

And regarding the purification of the parah (Parah Adumah, the red heifer) it is written "chukah" (a decree).

The Torah labels the red heifer a chok — a mitzvah without a graspable reason — setting the theme of this piece.

ובמדרש אמרתי אחכמה והיא רחוקה.

And in the Midrash: "I said I would become wise, but it is far from me."

The Midrash applies Shlomo HaMelech's words (Koheles 7:23) to the parah — even his wisdom could not reach the reason behind it.

כי כל טומאת מת נעשה ע"י חטא הראשון מעה"ד.

For all tumah (impurity) of death came about through the first sin, from the Eitz HaDaas (Tree of Knowledge).

Death itself, and the impurity it brings, entered the world only through Adam's eating from the Tree of Knowledge.

והייתם כאלקים יודעי טוב ורע.

"And you shall be like Elokim, knowing good and evil."

The nachash's enticement was precisely the desire to "know" — to grasp everything by one's own intellect.

ולכן הטהרה א"א להיות רק על ידי ביטול הדעת כמו שהי' רצונו ית' שלא יאכל מעץ הדעת והי' נמשך אחר החיים בלי תערובות.

Therefore the taharah (purification) can come about only through bittul of one's da'as (intellect), just as it was Hashem's ratzon (will) that man should not eat from the Eitz HaDaas, but rather be drawn after [the Tree of] Life without admixture.

Since the impurity stemmed from grasping at independent knowledge, its remedy is the opposite: nullifying one's own reasoning, as Hashem originally wanted man to cling to pure life without the "mixture" of the Tree of Knowledge.

לכן שטן ואוה"ע מונין את ישראל מה טעם יש בה.

Therefore the Satan and the nations of the world taunt Yisrael: what reason is there in it?

Precisely because this mitzvah demands surrender of intellect, the Satan and the nations mock Bnei Yisrael for keeping a law they cannot explain.

שרוצון לפתות את בנ"י כנחש הקדמוני להיות רוצין להבין הכל עפ"י שכל השפל של האדם.

For they wish to entice Bnei Yisrael, like the primordial nachash, to want to understand everything according to the lowly intellect of man.

The taunt is really the same seduction as the original snake's: insisting that nothing be accepted unless it fits human reason.

אבל בנ"י נמשכין באמונתם אחר רצונו ית' בלי טעם ומזה זוכין אל הטהרה כנ"ל:

But Bnei Yisrael, in their emunah, are drawn after Hashem's will without a reason, and through this they merit the taharah, as above.

By following Hashem's will out of pure faith rather than demanding explanations, Bnei Yisrael achieve exactly the bittul of intellect that purifies them from the impurity of death.

Summary: The red heifer is a chok because the tumah of death entered through Adam's grasping at independent "knowledge"; therefore its purification can only come through bittul ha-da'as — surrendering one's own reasoning and following Hashem's ratzon with pure emunah, the very opposite of the nachash's enticement that the nations still echo when they mock the mitzvah.