The question then arises, “What sort of evidence would prove the efficacy of prayer?” The thing we pray for may happen, but how can you ever know it was not going to happen anyway? Even if the thing were indisputably miraculous it would not follow that the miracle had occurred because of your prayers. The answer surely is that a compulsive empirical proof such as we have in the sciences can never be attained.
Some things are proved by the unbroken uniformity of our experienced. The law of gravitation is established by the fact that, in our experience, all bodies without exception obey it. Now even if all the things that people prayed for happened, which they do not, this would not prove what Christians mean by the efficacy of prayer. For prayer is a request. The essence of request, as distinct from compulsion, is that it may or may not be granted. And if an infinitely wise Being listens to the requests of finite and foolish creatures, of course He will sometimes grant and sometimes refuse them. Invariable “success” in prayer would not prove the Christian doctrine at all. It would prove something much more like magic—a power in certain human beings to control, or compel, the course of nature.
C S Lewis, in The World's Last Night