What did Zola mention in the book Naturalistic Photography?

Zola is both a famous writer and a photographer. Influenced by Zola's naturalism, Emerson, a representative figure of naturalistic photography, published a book "Naturalistic Photography" in 1889, arguing that photography should have its own independent language, and photography is an art combining art and science, and predicting that photography will become an independent art, combining documentary and artistic naturalism. Naturalistic photography attaches importance to the feeling of nature and advocates exploring themes in the actual natural environment.