Carl Friedrich Keil and Franz Delitzsch Old Testament Commentary

"And now," sc., because what was announced in Deu 31:16-18 would take place, "write you this song." "This" refers to the song which follows in ch. 32. Moses and Joshua were to write the song, because they were both of them to strive to prevent the apostasy of the people; and Moses, as the author, was to teach it to the children of Israel, to make them learn it, that it might be a witness for the Lord (for Me) against the children of Israel. "This" is defined still further in Deu 31:20, Deu 31:21 : if Israel, through growing satisfied and fat in its land, which was so rich in costly good, should turn to other gods, and the Lord should visit it in consequence with grievous evils and troubles, the song was to answer before Israel as a witness; i.e., not only serve the Lord as a witness to the people that He had foretold all the evil consequences of apostasy, and had given Israel proper warning (Knobel), but to serve, as we may see from Deu 31:20, Deu 31:21, and from the contents of the song, as a witness, on the one hand, that the Lord had conferred upon the people so many benefits and bestowed upon them such abundant blessings of His grace, that apostasy from Him was the basest ingratitude, for which they would justly be punished; and, on the other hand, that the Lord had not rejected His people in spite of the punishments inflicted upon them, but would once more have compassion upon them and requite their foes, and thus would sanctify and glorify Himself as the only true God by His judgments upon Israel and the nations. The law, with its commandments, promises, and threats, was already a witness of this kind against Israel (cf. Deu 31:26); but just as in every other instance the appearance of a plurality of unanimous witnesses raises the matter into an indisputable truth, so the Lord would set up another witness against the Israelites besides the law, in the form of this song, which was adapted to give all the louder warning, "because the song would not be forgotten out of the mouths of their seed" (Deu 31:21). The song, when once it had passed into the mouths of the people, would not very readily vanish from their memory, but would be transmitted from generation to generation, and be heard from the mouths of their descendants, as a perpetual warning voice, as it would be used by Israel for God knew the invention of the people, i.e., the thoughts and purposes of their heart, which they cherished (עשׂה used to denote the doing of the heart, as in Isa 32:6) even then before He had brought them into Canaan. (On Deu 31:20, vid., Deu 7:5; Deu 9:5, and Exo 3:8.) - In Deu 31:22 the result is anticipated, and the command of God is followed immediately by an account of its completion by Moses (just as in Exo 12:50; Lev 16:34, etc.). - After this command with reference to the song, the Lord appointed Joshua to the office which he had been commanded to take, urging him at the same time to be courageous, and promising him His help in the conquest of Canaan. That the subject to ויצו is not Moses, but Jehovah, is evident partly from the words themselves, "I will be with thee' (vid., Exo 3:12). (Note: Knobel's assertion (on Num 27:23) that the appointment of Joshua on the part of Moses by the imposition of hands, as described in that passage, is at variance with this verse, scarcely needs any refutation. Or is it really the case, that the installation of Joshua on the part of God is irreconcilable with his ordination by Moses?)