From Lewis' essay "The Efficacy of Prayer":
"Can we believe that God ever really modifies His action in response to the suggestions of man? For infinite wisdom does not need telling what is best, and infinite goodness needs no urging to do it. But neither does God need any of those things that are done by finite agents, whether living or inanimate. He could, if He chose, repair our bodies miraculously without food; or give us food without the aid of farmers, bakers, and butchers; or knowledge without the aid of learned men; or convert the heathen without missionaries. Instead, He allows soils and weather and animals and the muscles, minds, and wills of men to cooperate in the execution of His will. "God," says Pascal, "instituted prayer in order to lend to His creatures the dignity of causality." But it is not only prayer; whenever we act at all, He lends us that dignity. It is not really stranger, nor less strange, that my prayers should affect the course of events than that my other actions should do so."