Zola's theories are often viewed as being somewhat naive and insignificant. He himself claimed no originality for his own theories, but any historical account of naturalism must take into account his beliefs. However simple, naive, or invalid they might seem, they had a tremendous effect upon the development of not just French literature, but upon world literature. Because of his theories, he became one of the most abused and most championed figures on the literary scene, and the degree to which he was vilified indicates the amount of influence he wielded.
For Zola, naturalism was the systematic, objective, and scientific extension of realism. The duty of the novelist then was to present as accurate a picture of life as possible. This was to be accomplished by having the novelist function as scientifically and as objectively as possible when presenting his material. The subject matter of art should include some representatives of the working class, and the purpose should be to present an understanding of the contemporary social milieu. Thus, many of Zola's novels have a social idea as their basis, which is then presented with scientific objectivity. That is, the working class is not romanticized or shown as existing in some sort of perfect idyll; rather, all the trials and difficulties are presented in realistic terms.
Like the other naturalists, Zola demanded a great degree of verisimilitude in his novels, but he never claimed that the novel should be purely photographic. He once explained that art is a "corner of reality seen through a temperament" (from Mes Haines). The value of verisimilitude lies in the fact that, through exact descriptions, the artist could present his subject more accurately and consequently be more truthful to his subject.
The ultimate aim of the novelist is to be true to his subject matter and to present some basic truths about life. Only by giving the reader an accurate rendition of life could the artist be absolutely faithful to life. Any other form of writing would distort life, and the novelist then could not serve a useful purpose in his society.
When the artist presents his subject faithfully, then, for Zola, the character would be shown as a product of his environment and of his heredity. People's actions, like those of a beast, are controlled by the traits they inherited and by the environment from which they sprung. Consequently, throughout Nana, Zola constantly makes references to the gutters from which Nana came and how, unconsciously, she was attempting to drag all of society back to those gutters.
Even though there are none or few pure naturalists writing today, Zola's influence on the development of the novel has been tremendous. For example, he opened up new subject matter for the novelists and supported his use of lurid subject matter with philosophical principles.