The central theme of The Book of Merlyn is that by learning to pay attention to the natural world, humankind can learn how to avoid war and save itself from extinction. Although White was not fully conscious of this theme until he had almost completed the series, he came to think of it as the central theme of the entire Once and Future King. In 1940, he wrote to a friend that he intended to add a fifth volume to his Arthurian series because he had "suddenly discovered" that the central theme of Morte Darthur was to find "an antidote to war."

The assumption that underlies White's theme is that animals are more civilized than humans. Merlyn, the title character of the novel and the spokesperson for White's more cynical ideas, doesn't seem to think the chances for improving the human race are very good...