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.
