Christian fiction is sometimes harder to define than Christian non-fiction. Christian themes are not always explicit. Some Christian fiction, such as that of C. S. Lewis, draws on the allegorical writings of the past. There can also be argument as to whether the works of a Christian author are necessarily Christian fiction. For example, while there are undoubted Christian themes within J. R. R. Tolkien's "The Lord of the Rings", they are always kept below the surface. Other possible examples of Christian fiction include the works of G. K. Chesterton and George Macdonald.