In the 19th century, poet and free love advocate Algernon Charles Swinburne wrote a book on Blake drawing attention to the above motifs in which Blake praises "sacred natural love" that is not bound by another's possessive jealousy, the latter characterised by Blake as a "creeping skeleton". Swinburne notes how Blake's "Marriage of Heaven and Hell" condemns the hypocrisy of the "pale religious letchery" of advocates of traditional norms. Another 19th-century free love advocate, Edward Carpenter (1844–1929), was influenced by Blake's mystical emphasis on energy free from external restrictions.