An impure intention is one that yields to the will of God while retaining a preference for my own will. It divides my will from His will. It gives me a choice between two advantages: one in doing His will and one in doing my own. An impure intention is imprudent, because it weighs truth in the balance against illusion; it chooses between a real and an apparent good as if they were equal.Oh yes, I have a lot of impure intention. Take my money (please) for example: I give some to God's purposes, but the rest I want to use for my own. Or consider time—rather, attention: I pay attention to the things of God, like prayer, meditation, or thinking about how to serve and edify others (or myself for that matter). But after that I want to pay attention to what I want to pay attention to: writing some code, reading an "escape" novel, going on a bike ride, etc.Merton, No Man Is an Island 4.4 (pp. 54-55)
What does Merton mean when he talks about a real vs an apparent good? Simply that “True happiness is not found in any other reward than that of being united with God. If I seek some other reward besides God himself, I may get my reward but I cannot be happy.” (Merton, op. cit., 4.3, p. 54). These illusory rewards—the "vain things that charm me most" but cannot really satisfy—these are my false gods, my idols. They're only illusions, they don't satisfy, so why can't I just stop pursuing them? Like the Israelites, I hear the Lord calling me to "lie down in green pastures" (Psalm 23:2) and rest in him, to drink from "the spring of living water" that is God himself (Jeremiah 2:13).
"Do not run until your feet are bare and your throat is dry," he says to them in Jeremiah 2:25. And like them, I sometimes reply, "It's no use! I love foreign gods, and I must go after them." What is the cure to this folly? How can they, how can I, develop a pure intention? How can my corrupt thinking be corrected so that God's will is really mine? How can I surrender more fully to him?
It's not complicated, which is not to say it's easy. And it takes a long time. Basically it's spiritual formation, transformation, discipleship. It's the disciplines: praying God to change me, remembering and thinking about the truth (in the Bible, in the lives of those around me, in my own life): that "only in God is my soul at rest; in him is my salvation." (Psalm 62:1) Indeed, who will save me from the body of this death? Thanks be to God (Romans 7:24-25). Merton adds this word:
None of these things can be done without prayer, and we must turn to prayer first of all, not only to discover God's will but above all to gain the grace to carry it out with all the strength of our desire.And as it says in Hebrews 4:16, Let us then approach the throne of grace with confidence, so that we may receive mercy and find grace to help us in our time of need.Merton, op. cit. 4.9 (p. 61)