Commentary for Deuteronomy 21:4

אל נחל איתן UNTO A VALLEY WHICH IS איתן — which is hard; i.e. one that has never been tilled (Sifrei Devarim 207:2; Sotah 45b).

AND THE ELDERS OF THAT CITY SHALL BRING DOWN THE HEIFER UNTO A VALLEY ‘EITHAN’ — “which is ‘hard,’ one that has never been tilled.” This is Rashi’s language. And our Rabbis have said³⁶³Sotah 46b. that the field becomes forbidden from ever being tilled or sown. If so, [the words which is neither tilled nor sown]³⁶⁴In Verse 4 before us. are admonitions that he who sows or cultivates [a rough valley in which a heifer’s neck has been broken] transgresses this negative commandment. The meaning of the verse is thus that the elders should take forth the heifer to [what will forever become] “a rough valley” because it will never again be tilled or sown. The meaning of the expression And the priests the sons of Levi shall come near³⁶⁵Verse 5. is in order to recite Forgive, O Eternal, Thy people Israel, whom Thou hast redeemed etc.³⁶⁶Verse 8. And the purport of the verses is as follows: And the priests the sons of Levi shall come near;³⁶⁵Verse 5. and all the elders of that city, who are nearest unto the slain man, shall wash their hands;³⁶⁷Verse 6. and they shall speak and say: ‘Our hands have not shed this blood.’³⁶⁸Verse 7. The washing of hands is to be done only by some of the [elders] mentioned; so also their speaking [‘Our hands have not shed etc.’] is done by some of them, similar to what is stated, And thou shalt eat before the Eternal thy G-d etc. the tithe of thy corn, of thy wine, and of thine oil, and the firstlings of thy herd etc.³⁶⁹Above, 14:23. [meaning “he who is fit to eat,” for it cannot mean that all are to eat, for the Second Tithe may be eaten by all ritually clean Israelites, while the firstlings are eaten only by the priests; here, too, the washing of hands and speaking are done only by some, but not all, of the elders].
Now on the matter of breaking the heifer’s neck Rabbi Abraham ibn Ezra explained³⁶⁸Verse 7. that G-d commanded it to be done for the city which is nearest unto the slain man³⁷⁰Verse 3. because had they not committed a similar sin, it would not have happened that a man be killed near them, and G-d’s thoughts are infinitely profound to us. But the Rabbi [Moshe ben Maimon] said in the Moreh Nebuchim³⁷¹Guide of the Perplexed III, 40. Ramban follows Al Charizi’s translation. In Ibn Tibbon’s text the wording is quite different. that the reason for the breaking of the heifer’s neck is to discover the murderer and to cleanse [the guilt for the innocent] blood. In most cases the murderer comes from the place which is round about him that is slain,³⁷²Verse 2. and when the elders go out and engage in the measuring of the cities, and the elders of the nearest city then bear witness before the Creator that they were not negligent in maintaining and guarding the roads, and that they do not know who killed this man, and as the matter is investigated, the elders gather and bring the heifer, people increasingly speak about it. Then perhaps the matter [of the murderer’s identity] will be solved. The Rabbis have already said³⁷³Yerushalmi Sotah IX, 1. that even if a maidservant comes and declares that a certain person committed the murder, the heifer is not killed [despite the fact that such testimony is not sufficient to convict]. And if the murderer is known and they are silent about him, yet call upon the Creator as their witness that they do not know him, this would be a great wickedness, and whoever knows the slightest thing about it will come and tell. Thus it [the murderer’s identity] will become public knowledge, and he will be killed either through the court, or the king, or the avenger of the blood. This law receives added force because the place in which the heifer’s neck is broken may never be tilled or sown. Those who see [the field] recognize it and discuss it. [Thus far are the words of Rabbi Moshe ben Maimon.] Hence according to this reason there is a benefit in this rite. But the act [i.e., the breaking of the heifer’s neck] is not a pleasing one in itself and it would have been more fitting that it be done in a good field fit for sowing that could be recognized by those who see it, for in “a hard valley,” the reason for not being tilled, will not be obvious [because people will assume that it is left barren due to its stony ground]! In my opinion the reason for it is similar to that of the offerings which are done outside [of the Sanctuary Court], such as the goat sent [to Azazel]³⁷⁴Leviticus 16:22 and 26. and the Red Heifer.³⁷⁵Numbers 19:3. Therefore the Rabbis have counted³⁷⁶This is so counted by Rambam, at the end of Hilchoth Me’ilah. See my Hebrew commentary, p. 440, Note 23. the commandment of breaking the heifer’s neck among the chukim [“the statutes” — the class of commandments for which we do not know the reasons].
Ki Theitzei

אל נחל איתן, where people do not normally walk.