Deuteronomy 31:15-21

Verse 16

Behold, thou shalt sleep with thy fathers - שכב shocheb, thou shalt lie down; it signifies to rest, take rest in sleep, and, metaphorically, to die. Much stress cannot be safely laid on this expression to prove the immortality of the soul, or that the people in the time of Moses had a distinct notion of its separate existence. It was, however, understood in this sense by Jonathan ben Uzziel, who in his Targum paraphrases the word thus: "Thou shalt lie down in the dust with thy fathers; and thy soul (נשמתך nishmethach) shall be laid up in the treasury of the life to come with thy fathers."
Verse 18

I will surely hide my face - Withdraw my approbation and my protection. This is a general meaning of the word in Scripture.
Verse 19

Write ye this song - The song which follows in the next chapter. Things which were of great importance and of common concern were, among the ancients, put into verse, as this was found the best method of keeping them in remembrance, especially in those times when writing was little practiced. Even prose was sometimes sung. The history of Herodotus was divided into Nine books, and each inscribed with the name of one of the Nine Muses, because these books were anciently sung. Homer is reported to have sung his poems through different Greek cities. Aristotle observes that anciently the people sung their laws. And Cicero observes that it was a custom among the ancient Romans to sing the praises of their heroes at the public festivals. This was the case among the northern inhabitants of Europe, particularly in Ireland and Scotland; hence the Gaelic poetry of Ossian and others. See Dodd; and see the note on Exo 15:1, where the subject is largely treated.
Verse 21

This song shall testify against them - Because in it their general defection is predicted, but in such a way as to show them how to avoid the evil; and if they did not avoid the evil, and the threatened punishment should come upon them, then the song should testify against them, by showing that they had been sufficiently warned, and might have lived to God, and so escaped those disasters.
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