I promise that I know you very well
Your eyes never lie even if they fell
Out the sky and your optics turn stale
Where they mold that's green
Kendrick knows his friend so well that he can read his intentions through the look in his eyes. He cannot deceive him easily.
Also, Kendrick utilizes the word “lie” in both ways, creating a double entendre. Once your eyes fall from being focused on God (who is constant and unchanging) onto something as trivial as money (which comes and goes), they will never remain in a position of inactivity (to lay). “Where they mold that’s green” is symbolic of one becoming jaded. Jade is an ornamental stone that is green in hue.
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