“…This is the decree of Torah that G-d commanded to say. Speak to Bnei Yisrael and have them take to you, a red heifer…” – Bamidbar 19:2

זֹאת חֻקַּת הַתּוֹרָה אֲשֶׁר צִוָּה ה’ לֵאמֹר דַּבֵּר אֶל בְּנֵי יִשְׂרָאֵל וְיִקְחוּ אֵלֶיךָ פָרָה אֲדֻמָּה – במדבר יט, ב

The Torah commands us to use the ashes of a parah adumah, a red heifer, for the ritual purification of people or articles that contracted impurity from a dead corpse. The mitzvah of Parah Adumah is classified by the Torah as a chok, a decree that transcends rational reason. Indeed, King Shlomo, the wisest of all men, said regarding this mitzvah, “All of the Torah’s commandments I have comprehended. But the chapter of the red heifer, though I have examined it, questioned it and searched it out—‘I thought to be wise [in it], but it is distant from me’ (Koheles 7:23)” (see Bamidbar Rabbah 19:5).

Nevertheless, despite this mitzvah’s uniqueness as the ultimate chok, the Torah introduces its laws with the words, “This is the decree of the Torah”—associating the entire Torah with this inexplicable mitzvah. In doing so, the Torah teaches us that the inexplicableness that characterizes the mitzvah of Parah Adumah is, in fact, common to all the mitzvos.  For the mitzvos are in essence expressions of G-d’s will; i.e., what G-d wills simply because He so wills. Their relevance is therefore not defined by logic and reason—even G-dly reason. True, G-d desired that some of the mitzvos should also “descend” to the realm of reason, and He therefore gave them rational significance as well. At their core, however, even the rational mitzvos are chukim, decreed by G-d’s will; their ultimate purpose transcends all reason.

This explains why G-d did not reveal the reason for the Parah Adumah even to the wisest of men. If every aspect of the Torah were logically explicable, then the always-rational Jew would be at a loss when the fulfillment of a mitzvah would require self-sacrifice, a demand that is inherently irrational. The mitzvah of Parah Adumah is therefore inherently inexplicable and is called “the decree of the Torah,” because the chok element of Parah Adumah serves as the basis for our uncalculated devotion to all the Torah’s commands.

—Likutei Sichos, vol. 18, pp. 230-232