What is meant by "Falsehood cannot approach it from before it or from behind it," and what is the meaning of "from before it" and "from behind it"?

The exegetes have mentioned several interpretations regarding the Quranic verse: "Falsehood cannot approach it from before it or from behind it; [it is] a revelation from a [Lord who is] Wise and Praiseworthy" [Fussilat:42]. Among these interpretations are: that denial cannot come to it, or that Satan cannot diminish any truth from it nor add any falsehood to it, or that falsehood is incapable of adding a letter to it or removing a letter from it.

Al-Tabari favored the interpretation that its meaning is: no falsehood-holder can alter it with his guile or by changing any of its meanings – and that is its being approached from before it; nor can he append to it what is not part of it – and that is its being approached from behind it.