Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
I am an hundred and twenty years old - The 40 years of the wandering had passed since Moses, then 80 years old, "spake unto Pharaoh" (Exo 7:7; Compare Deu 34:7).
I can, no more go out and come in - Render I shall not longer be able to go out and come in: i. e., discharge my duties among you. There is no inconsistency with Deu 34:7. Moses here adverts to his own age as likely to render him in future unequal to the active discharge of his office as leader of the people: the writer of Deu 34:1-12, one of Moses' contemporaries, remarks of him that up to the close of life "his eye was not dim, nor his natural force abated" Deu 31:7; i. e. that he was to the last, in the judgment of others, in full possession of faculties and strength.
Moses hands over to Joshua that office as leader of the people, to which he had already been designated Deu 1:38; Num 27:23. He assigns also to the Levitical priests and the elders, as the ecclesiastical and civil heads of the nation, the responsibility of teaching the law and enforcing its observance Deu 31:10-13. Both these were symbolic acts, designed to mark the responsibility of the parties concerned after the death of Moses.
Compare the marginal references. It is not to be supposed that the whole of the Pentateuch was read, nor does the letter of the command require that it should be so. This reading could not be primarily designed for the information and instruction of the people, since it only took place once in seven years; but was evidently a symbolic transaction, intended, as were so many others, to impress on the people the conditions on which they held possession of their privileges and blessings.
The transaction recorded in these verses may be regarded as the solemn inauguration of Joshua to the office to which he had some time before Num 27:22 been called, and his recognition in it by God, which were manifested by his being summoned into the tabernacle with Moses while the Lord appeared in the pillar of cloud (compare Num 11:25; Num 12:5).
The future apostasy of the people is announced in the presence of Joshua that the latter might be fully aware of the danger and strive in his day to avert it. This he faithfully did (compare Jos 24:31); but we find him in his own last address to Israel repeating Jos 23:15-16 the self-same prediction and warning.
A witness for me against them - i. e., an attestation from their own mouths at once of God's benefits, their own duties, and their deserts when they should fall away. Being in verse it would be the more easily learned and kept in memory. The use of songs for such didactic purposes was not unknown to the legislators of antiquity. Compare also the advice of Paul, "teaching and admonishing one another in psalms and hymns and spiritual songs" Col 3:16.
He gave - i. e., the Lord gave.
Moses completes the writing out of the book of the Law, and directs it to be placed by the ark of the covenant.
The "book" here spoken of would contain the whole Pentateuch up to this verse, and be "the Book of Moses," called generally by the Jews "the Law" (compare Mat 22:40; Gal 4:21).
The Levites, which bare the ark - i. e., as in Deu 31:9, "the priests the sons of Levi." The non-priestly Levites could not so much as enter the sanctuary or touch the ark (compare Num 4:15). Though in the journeys through the wilderness the ark was borne by the non-priestly Kohathites, yet on occasions of a more solemn and public character it was carried by the priests themselves (Jos 3:3 ff, Jos 4:9-10; Jos 6:6, Jos 6:12; Jos 8:33; Kg1 8:3).
Put it in the side of the ark - Rather, by the side of the ark. The two tables of the Decalogue were in the ark Kg1 8:9; the Book of the Law was to be laid up in the holy of holies close by the ark of the covenant, probably in a chest. Compare Kg2 22:8.
How much more after my death - Hence, Deu 31:24 and the rest of the book (with the exception of the song, Deu 31:19) must be regarded as a kind of appendix added after Moses' death by another hand; though the Blessing Deut. 33 is of course to be regarded as a composition of Moses.