Halakhah к Бамидбар 19:9

LAWS CONCERNING THE RED HEIFER.
These comprise two affirmative precepts: 1) the Red Heifer; 2) the uncleanliness of the waters of sprinkling (Num. 19:9-22) and of the mode in which these are to be used for purification.

And that is that He commanded us to process the red heifer, so that its ashes be ready for one who needs it for purification from the impurity of a corpse - as He said, "and it shall be for the congregation of the Children of Israel" (Numbers 19:9). And the laws of this commandment have already been explained in Tractate Parah. (See Parashat Chukat; Mishneh Torah, Red Heifer 1-15.)

The commandment of the red heifer: That Israel was commanded to burn the red heifer so that its ashes will be ready for anyone who needs it to be purified from the impurity of the dead, as it is stated (Numbers 19:2), "Speak to the Children of Israel and they shall take to you a red heifer," and it is written below this (Numbers 19:9), "It will be a safeguard for the Children of Israel." Even though my heart has given me the gumption to write hints of the simple reasons for the previous commandments, with the excuse that [this] work is to instruct my son and his young friends, may God protect them; on this commandment my hands are weak and I am afraid to open my mouth about it at all, since I have seen that our Rabbis, may their memory be blessed, spoke at length regarding the depth of its secret and greatness of its content; to the point that they said (Bemidbar Rabbah 19, Midrash Tanchuma 4:6:6) that King Solomon was able through his great wisdom to understand all the reasons of the Torah, except for this - as he stated about it (Ecclesiastes 7:23), "I have said that I will understand, but it is far from me." They also said in the Midrash Tanchuma 4:6:8, "Rabbi Yose BeRebbi Chanina says, 'The Holy One, blessed be He, said to Moshe, "To you I will reveal the reason for the red heifer, but not to others."'" And there are many other similar statements. And now, the listener should not think that the matter of its secret and the matter of its being an arational commandment (chok) is that the ashes affect purification, as one will find a similar [process] with other sacrifices for the person with a discharge or a new mother, whose purification is completed by the offering of their sacrifices.