Charles H. Spurgeon on Psalms 17:3

"I am purposed that my mouth shall not transgress." Wherefore, if thou be upon a mountain, look not backward again unto Sodom as Lot's wife did; if thou be within the ark, fly not out again into the world as Noah's crow did; if thou be well washed, return not again to the mire as the hog doth; if thou be clean, run not again to thy filth, as the dog doth; if thou be going towards the land of Canaan, think not on the flesh-pots of Egypt; if thou be marching against the host of Midian, drink not of the waters of Harod; if thou be upon the housetop, come not down; if thou have set thy hand to the plough, look not behind thee; remember not those vices which are behind thee. —Thomas Playfere.

Verses 3, 4, 5. Where there is true grace, there is hatred of all sin, for hatred is (Greek). Can a man be resolved to commit what he hates? No, for his inward aversion would secure him more against it than all outward obstacles. As this inward purpose of a good man is against all sin, so more particularly against that which doth so easily beset him. David seems in several places to be naturally inclined to lying, but he takes up a particular resolution against it: (verse 3), "I am purposed that my mouth shall not transgress;" (Heb.) —I have contrived to waylay and intercept the sin of lying when it hath an occasion to approach me. A good man hath not only purposes, but he endeavours to fasten and strengthen those purposes by prayer; so David (verse 5), "Hold up my goings in the paths, that my footsteps slip not." He strengthens himself by stirring up a liveliness in duty, and by avoiding occasions of sin; (verse 4), "I have kept me from the paths of the destroyer;" whereas, a wicked man neither steps out of the way of temptation, nor steps up to God for strength against it. —Stephen Charnock.