Inspiration for the series is taken from multiple sources; in addition to adapting numerous traditional Christian themes, the books freely borrow characters and ideas from Greek and Roman mythology as well as from traditional British and Irish fairy tales. The books have profoundly influenced adult and children's fantasy literature since World War II. Lewis's exploration of themes not usually present in children's literature, such as religion, as well as the books' perceived treatment of issues including race and gender, has caused some controversy.