The two methods by which we are allowed to produce events may be called work and prayer. ... What we do when we weed a field is not quite different from what we do when we pray for a good harvest. But there is an important difference all the same.
You cannot be sure of a good harvest whatever you do to a field. But you can be sure that if you pull up one weed that one weed will no longer be there. ... The kind of causality we exercise by work is, so to speak, divinely guaranteed, and therefore ruthless. By it we are free to do ourselves as much harm as we please. But the kind which we exercise by prayer is not like this; God has left Himself a discretionary power. Had He not done so, prayer would be an activity too dangerous for man and we should have the horrible state of things envisaged by Juvenal: "Enormous prayers which Heaven in anger grants."
Prayers are not always -- in the crude, factual sense of the word -- "granted". This is not because prayer is a weaker kind of causality, but because it is a stronger kind. When it "works" at all it works unlimited by space and time. That is why God has retained a discretionary power of granting or refusing it; except on that condition prayer would destroy us. It is not unreasonable for a headmaster to say, "Such and such things you may do according to the fixed rules of this school. But such and such other things are too dangerous to be left to general rules. If you want to do them you must come and make a request and talk over the whole matter with me in my study. And then -- we'll see."
--C. S. Lewis, "Work and Prayer"