Deutéronome 33:6

Let Reuben live in this world, nor die the second death which the wicked die in the world to come; and let his youths be numbered with the young men of his brethren of Beth Israel.

May Reuven live He begins with the eldest, as is customary. He does not mention Shimon, either because of the incident of Baal Peor [Numbers 25: 3], in which the worshipers were Shimonites, as evidenced by their decline in numbers [Numbers 26: 14] [cf. Numbers 1: 23], as well as by the fact that it was their chieftain who was killed [Numbers 25: 14]. or He follows Yaaqov’s example, who did not bless Shimon or Levi. Out of respect for Aaron, his tribe was excepted, borne up on the coattails of Aaron; but Shimon lacked a great man of equivalent stature. If one should object, and ask, Why, then, did Moshe bless Reuven? — the answer is that Yaaqov did bless Reuven, the blessing being “you shall not stand out” [Genesis 49: 4], i.e., that he should be as one of his brethren. After Reuven, he mentions Y e hudah, who was the standard-bearer, and the first to go into battle. Then Levi, and then Benjamin, on account of the Levites’ dwelling in Jerusalem, which lay between the territory of Y e hudah and the territory of Benjamin, as it is written [cf. Joshua 18: 28 , Judges 1: 8]. Then the tribe of Joseph, in order not to postpone them, having already given precedence to his younger brother. Then Zevulun, on account of his going into war [Judges 5: 18], and then Yissakhar. After completing the children of the major wives, he begins with Gad, who encamped under the standard of two children of the major wives, and then Dan, since he was a standard-bearer, and then Naftali, because he was greater than Asher.

The phrase May Reuven live is a supplication.

and not die for it is possible that the tribe could survive a finite number of years, like a man, and then die.

and his men be numbered i.e., and his men not be numbered, as in “I have not learned knowledge, [nor] understood holiness” [Proverbs 30: 3] (see also the particle “as” in “as Almighty God” [Exodus 6: 3] [comment on Exodus 6: 3]), as I have explained in numerous places — for it is possible for the tribe to survive indefinitely, and still remain few in number. Anything that can be numbered is scanty (compare, “my men are numbered” [Genesis 34: 30]).

LET REUBEN LIVE. Rabbi Abraham ibn Ezra commented that “he began with Reuben because he was the firstborn. He did not mention Simeon because of the sin of [Baal] Peor, because the worshippers were Simeonites, the proof being their numbers,, 26:14), a decrease of 37,100 people. In the opinion of Ibn Ezra the decrease would be accounted for by the 24,000 people that died in the plague because of the Peor affair (ibid., 25:9) thus indicating that the idol-worshippers were exclusive of that tribe. On this point, Ramban will comment that even assuming that all 24,000 who died in the plague were from the tribe of Simeon, there are still 13,100 people unaccounted for, a discrepancy that cannot be explained by Ibn Ezra’s theory. and their prince, too, being killed [in the affair].” In my opinion, the numbers are no proof because over thirteen thousand are still unaccounted for, aside from those who died in the plague., 26:14), a decrease of 37,100 people. In the opinion of Ibn Ezra the decrease would be accounted for by the 24,000 people that died in the plague because of the Peor affair (ibid., 25:9) thus indicating that the idol-worshippers were exclusive of that tribe. On this point, Ramban will comment that even assuming that all 24,000 who died in the plague were from the tribe of Simeon, there are still 13,100 people unaccounted for, a discrepancy that cannot be explained by Ibn Ezra’s theory. In addition, other tribes as well diminished in population, for the sons of Gad lost five thousand,, 26:18) — a decrease of 5,150. — And so perhaps we should assume that these deaths resulted from the plague because of Peor. and the sons of Ephraim, eight thousand!, 26:37) — a decrease of 8000. Moreover, from the verse which states, And Israel attached himself unto Baal-peor, and it is said again, Take all the chiefs of the people, Verse 4. it would appear that there were some among them from all the tribes, and that all their judges passed sentence on them. And similarly He said, so that I consumed not the children of Israel in My jealousy., Verse 11. Now do not rejoin against me from the affair of Achan where He said, Israel hath sinned. — For the significance thereof see Ramban further. [although it was but a single individual who sinned; similarly we can say that it was the tribe of Simeon alone that sinned in the affair of Peor yet Scripture describes it as if the entire nation sinned. This …

יחי ראובן , even though the tribe of Reuven had chosen to live in a ritually unclean country (East bank of the Jordan), a region which is not really part of the “Holy Land,” as per Joshua 25,19 “although the land of your heritage is an unclean land, etc.,” and as a result of this they are not really entitled to eternal life in the hereafter, Moses prays that his (Reuven’s) heart may live on forever,

ואל ימות ויהי מתיו מספר, may he not die out in this life on earth so that his numbers will become too insignificant. (Yaakov already described people who are few in numbers as being in danger of being wiped out, Genesis 34,30) Moses prays that the tribe of Yehudah be included in the blessing of Deut. 11,21 “so that you will enjoy many years on the earth, etc.”

יחי ראובן ואל ימות, “may Reuven live and not die.” When the tribe of Reuven will fulfill its promise and march as vanguard in front of the army conquering the land of Canaan, (together with the tribe of Gad and half the tribe of Menashe, as per Numbers 32,20-42) they will not suffer a single casualty during that campaign.

ויהי מתיו מספר, “when counted afterwards they will not have diminished in number.” This comment refers to the previous promise of Moses that they will not suffer casualties by fulfilling their promise. In fact, they will continue to grow in numbers.