Rise mightily against the first actings of thy distemper, its first conceptions; suffer it not to get the least ground. Do not say, ‘Thus far it shall go and no farther.’ If it have allowance for one step it will take another. It is impossible to fix bounds to sin. It is like water in a channel—if it once break out, it will have its course. Its not acting is easier to be compassed than its bounding.

John Owen
The Mortification of Sin (Banner of Truth; abridged edition, 2004), 55