The dominant philosophy in a novel is the primary message the writer is trying to convey to his or her readers. A novel, as opposed to a short story, may contain multiple messages across many chapters. However, rising to the top amongst all of these is the main one that the writer wants readers to grasp and carry with them in their minds - long after the actual reading of the novel is complete. The dominant philosophy can be a blatant message trumpeting it's way, chapter after chapter. It may be a more subdued message that creeps up on the reader as the novel develops, finally coming to full illumination at the conclusion of a story. The dominant philosophy in quality literary works is what people talk about later when they discuss the main point/main message of a novel.
In essence, the dominant philosophy is the underlying driving force of the novel. It influences plot, setting, characterization, dialogue, and even the structure of the novel. The dominant philosophy is something the writer always has in the back of his or her mind as they piece their novel together. It affects the writing decisions the author makes because the dominant philosophy is like a unifying element that helps keep the novel on track. When the writer adheres to the dominant philosophy they wish readers to grasp, they stay on the straight and narrow so-to-speak and do not, hopefully, meander off message.
Certain factors affect the dominant philosophy of a novel. The writer's own views, prejudices (if any), opinions, education, and cultural upbringing can affect the dominant philosophy of their work. The time period in which the writer is living may affect the dominant philosophy of their work. A writer who is totally objective may not let these factors affect the dominant philosophy of their novel; they may focus on what they believe should be the dominant philosophy of a main character in the story. They try to do this without their own judgements, and more, affecting the story, which can be difficult to do at times.