In his essay "A Defense of Naturalism," Roger Sherman Loomis declares,
...the Naturalist discards as obsolete three supernaturalistic concepts—Providence, absolute morality, and freedom of the will.
- The supernatural as only a development of the natural
Steinbeck's character Jim Casy exemplifies this concept. He has been a preacher, but he has abandoned the traditional role. Instead, he tries to serve people by working with them and by setting an example. He tells the Joads, "Maybe all men got one big soul everybody's a part of." He discards the idea of Jesus as savior and says that men must work together in order to attain anything. And, only reason enhanced by experience can direct people to the truth. Through such characters as Casy and Ma Joad and Tom in his novel The Grapes of Wrath, Steinbeck portrays man as a highly complex creature, whose nature includes, besides his natural urges, a social and artistic instincts as well as reason. If all these factor are in harmony, happiness can be attained.
Steinbeck's novel presents "the environment and nature’s effects on social history." The great French Nauralist, Emile Zola, explains the idea of determinism as the theory that a person's fate is determined solely by heredity and environment. Many of Steinbeck's intercalary chapters portray this determinism. For example, in Chapter Five the description of the spokesman for the land owners informing the sharecroppers of their fates that they must leave the land on which their families have lived for generations. Even the owner men feel “caught in something larger than themselves” as the banks dictate to them. However, the Naturalist does not always make everyone a complete victim of fate; there are those who work together and are able to endure. The Joads, for example, represent the social group, the unit of strength that abides throughout, no matter the cost to them individually.
- Characters are fully developed early in the narrative in order that the reader may be an informed observer.
In The Grapes of Wrath, characters are described with much detail and are introduced early in the narrative. Tom Joad and Jim Casy are well developed by Chapter Four. And, Ma Joad and the rest of the Joad family is introduced and characterized by Chapter Eight.
The theme of "the cause of labor" is typically Naturalistic. Steinbeck's main objective in his narrative is the illustration of the dispossed Oklahoma farmers who must migrate and seek work where there none or where there is much injustice.
Another typical theme is that of "ordinary people in extraordinary circumstances." The Joads and others like them find themselves and all their worldly belongings traveling down unknown highways, alienated from friends and familiar surroundings as they search for work.
Naturalists employ symbolism, and Steinbeck utilizes the Joad family as representative of the larger "family of humanity." Especially poignant and meaningful is Rose of Sharon's giving literally of the "milk of human kindness" to the starving man. Another symbol is the land, which represents the people themselves and their histories.
The narrator of this novel maintains an objective attitude toward the characters, observing their behaviors in a scientific way of merely recording the details of what characters do and say in response to natural forces.