3You have tried my heart, you have visited me by night, you have tested me, and you will find nothing; I have purposed that my mouth will not transgress.
David says, I have purposed that my mouth will not transgress. He is saying that he has planned, decided, to say nothing sinful. He wants to maintain his claim to integrity, to be in the right.
3 You have tried my heart, you have visited me by night, you have tested me, and you will find nothing; I have purposed that my mouth will not transgress.