C. S. Lewis in The World's Last Night, quoted in Christianity Today, January 11, 1993, p. 34
Some things are proved by the unbroken uniformity of our experiences. 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 request. The essence of request, as distinct from compulsion, is that it may or may not be granted.