I've begun reading George Bernard Shaw's The Perfect Wagnerite, a book of commentary on Richard Wagner's famous Ring cycle. The book was published in 1899. In the preface to the third edition, however, Shaw writes of the knowledge necessary to understand the allegorical layers in the Ring:

... is an absolute blank to many persons who are highly susceptible to the musical qualities of Wagner's music and poetry, but have never reflected on human destiny at all, and have been brought up in polite ignorance of the infernal depths our human society descended to in the nineteenth century.

The preface was written in 1913. It could not have been written three years later.

How relative "infernal depths" are...