Other things are proved not simply by experience but by those artificially contrived experiences which we call experiments. Could this be done about prayer? I will pass over the objection that no Christian could take part in such a project, because he has been forbidden it: “You must not try experiments on God, your Master.” Forbidden or not, is the thing even possible?
I have seen it suggested that a team of people—the more the better—should agree to pray as hard as they knew how, over a period of six weeks, for all the patients in Hospital A and none of those in Hospital B. Then you would tot up the results and see if A had more cures and fewer deaths. And I suppose you would repeat the experiment at various times and places so as to eliminate the influence of irrelevant factors.
The trouble is that I do not see how any real prayer could go on under such conditions. “Words without thoughts never to heaven go,” says the King in Hamlet. Simply to say prayers is not to pray; otherwise a team of properly trained parrots would serve as well as men for our experiment. You cannot pray for the recovery of the sick unless the end you have in view is their recovery. But you can have no motive for desiring the recovery of all the patients in one hospital and none of those in another. You are not doing it in order that suffering should be relieved, you are doing it to find out what happens. The real purpose and the nominal purpose of your prayers are at variance. In other words, whatever your tongue and teeth and knees may do, you are not praying. The experiment demands an impossibility.
C.S. Lewis, “The Efficacy of Prayer,” originally from The World’s Last Night and Other Essays, here from The Essential C.S. Lewis (Touchstone, 1996) 379-380.