Reasons Beyond Nature

במדרש לך אני מגלה טעמי פרה ולאחרים חוקה

In the Midrash: "To you, [Moshe,] I am revealing the reasons of the parah [adumah, the red heifer], but to others it remains a chok [a decree whose reason is hidden]."

The parah adumah is the classic chok, a mitzvah performed in pure obedience whose reason is hidden, yet Hashem singled out Moshe Rabbeinu to receive its inner rationale even though for everyone else it stays concealed.

כי הטעמים הם למעלה מן הטבע

This is because the reasons [behind the mitzvos] are above nature.

The reasons of the mitzvos lie on a plane higher than the natural order, which is precisely why they remain hidden from ordinary, nature-bound perception.

לכן נאמר במד' כי אותם התינוקת שלא טעמו טעם חטא היו יודעין לדרוש מ"ט פנים טהור כו'

Therefore the Midrash relates that those very young children, who had not yet tasted the taste of sin, were able to expound forty-nine aspects [showing a thing to be] tahor (pure), and so forth.

Because these reasons transcend nature, it took children still untouched by the taint of sin, and thus still attached to that supernal level, to be able to derive the many pure aspects of this mitzvah.

ומרע"ה שהי' איש האלקים ממש והי' דבוק בתורה שבכתב נגלה לו טעמי פרה ולמטה היא חוקה וכ' ויקחו אליך ע"י שנתבטלו בנ"י אל הצדיק שהוא למעלה מן הטבע יכולין להשיג ג"כ בכלל הטעמים:

And Moshe Rabbeinu, who was literally an ish ha'Elokim (a man of Hashem) and was bound up (dveikus) with the Torah she'biksav (the Written Torah), had the reasons of the parah revealed to him, while below it remains a chok; and Scripture writes "And they shall take to you" (Bamidbar 19:2), meaning that through Bnei Yisrael nullifying themselves (bittul) to the tzaddik, who is above nature, they too can attain, in a general way, a share in grasping the reasons.

Moshe Rabbeinu reached this level through his utter dveikus to the Torah she'biksav and his stature as a true ish ha'Elokim, and the verse hints that ordinary Bnei Yisrael can share in that higher understanding when they nullify themselves to such a tzaddik who already stands above nature.

Summary: The Sfas Emes opens from the Midrash that Hashem revealed the reasons of the parah adumah to Moshe Rabbeinu while leaving it a chok for everyone else, and explains that this is because those reasons reside on a plane that is above nature, inaccessible to ordinary perception. For this reason it was specifically children who had not yet tasted the taste of sin, and so were still bound to that supernal level, who could expound the forty-nine pure aspects of the law. Moshe Rabbeinu attained these reasons through being literally an ish ha'Elokim and clinging in dveikus to the Torah she'biksav, even as the mitzvah remains a chok in the lower, natural realm. The deeper lesson the Sfas Emes draws from "And they shall take to you" is that Bnei Yisrael themselves, through bittul to the tzaddik who already stands above nature, can come to share in grasping these reasons that transcend the natural order.