This song... shall not be forgotten out of the mouths of their seed. — And it is not forgotten now. St. Paul made special use of it in the last days of the second Temple. This song is a favourite piece of Hebrew poetry to this day. Rashi observes: “This is a promise to Israel that the law shall not be utterly forgotten by their seed.”
I know their imagination. — Heb., yêtzer, the same word employed in Genesis 6:5; Genesis 8:21. It is the word commonly used in Rabbinical literature for the evil nature or good nature in any man. The nature which they are forming, or making, this day, would be a literal rendering of the sentence in this verse. And yet with all this, He made Balaam say, “He hath not beheld iniquity in Jacob nor seen perverseness in Israel” (Numbers 23:21). Comp. 1 Chronicles 28:9, “The Lord... understandeth all the imaginations of the thoughts,” and Psalms 103:14, “He knoweth our frame (yêtzer); He remembereth that we are dust.”