In Book VII, Hugo digresses to defend his use of slang. Slang, he concedes, is a horrible, pestilential language, but the novelist no more than the scientist can exclude any phenomenon from his field of inquiry, however unpalatable. Slang in its purest sense is the weapon of the have-nots against the establishment. Hence its preservation is of sociological interest. Furthermore, the study of slang is a means of curing the misfortune that has engendered it.
The spirit of slang, admittedly, is evil: "It is a dressing room in which the language disguises itself because it has some evil deed to do." But let us be merciful to it and to those who speak it, for human existence seems to indicate that none of us is free of guilt. The universal unhappiness of mankind seems to suggest that all of us are carrying the burden of divine retribution.
Slang, besides its ethical interest, has a literary value. It is the poetry of evil. It is a kind of geological formation whose numerous layers contain the fossils of various foreign words. It coins evocative expressions, it creates metaphors, it freely reshapes the language. Slang is dynamic, ever changing. It is the mirror of a soul, for a close study of it reveals the psychology of the underworld.
Victor Hugo is not the only nineteenth-century novelist to immortalize slang in his pages. Zola, Balzac, Dickens — all frequently use slang and dialect in their novels to add authenticity and realism to the speech of their characters. This was, however, still considered an innovation, and not always a desirable one. Many people in the nineteenth century were still horrified to find such "vulgar" language in a work of art, so Book VII is really a document setting forth Hugo's views on a lively literary controversy.
In the sixteenth century, the poets of the French Pléiade introduced a number of new dialectal and technical words into the French literary vocabulary in order to make the French language more flexible and more representative of the wider world of the Renaissance. The classical writers of the seventeenth century, however, adopted the doctrine that true art dealt only with the noble and the beautiful, and expressed it in universal and general terms. In accordance with this view, they purified literary language of most of its colorful "special" terminology, and writers of the eighteenth century narrowed the vocabulary yet further to the point of monotony. The early Romantics, in their revolt against classical strictures on art, rebelled against restrictions on vocabulary too.
Hugo was one of the leaders of this movement and almost single-handedly brought about a complete revolution in the concept of "poetic" vocabulary. "I have put the red bonnet of the Revolution on the old dictionary," he said, and in language he used any word that pleased him — exotic, learned, archaic, technical, or vulgar — as long as it conveyed his meaning more effectively. Here, he defends slang because it seems to him a colorful expression of the poetry of the people, the authentic voice of their courage and their defiant despair. Today, thanks to Hugo and others like him, there are no censored words in literature, and writers have a freer artistic vocabulary than ever before.