Holes is packed as tight with conflict as the dry dirt beds of Camp Green Lake are packed with dirt. The book's author, Louis Sachar, doesn't throw just one type of conflict at Stanley, the book's main character. Stanley struggles with all three of these types of conflict.
First, let's look at "Man vs. Self." No, this doesn't mean Stanley tries to arm-wrestle himself. It refers to an inner struggle that he must overcome. Stanley's biggest inner struggle is to believe in his own worth and strength. You can see Stanley struggling with this throughout the novel. For example, in Chapter 7, Stanley has trouble digging a hole. He blames himself for not being strong enough:
He glanced helplessly at his shovel. It wasn't defective. He was defective.
"Man vs. Man" is a bit easier to identify. Who is Stanley's biggest adversary in the novel—the one who works against him the most and poses the biggest threat? In this case, it's the Warden. This comes across in many scenes. One example is when the Warden puts a stop to the arrangement Stanley has made with Zero, in which he teaches Zero to read and Zero helps dig Stanley's holes:
"Okay, from now on, I don't want anyone digging anyone else's hole," said the warden. "And no more reading lessons."
Stanley also struggles against society. After all, it's society that has allowed Camp Green Lake to exist. Society has further failed to ensure the safety of the boys who are sent there. It's society that failed to take care of Zero, who becomes very important to Stanley.
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All three of those types of conflicts are present in Holes. The boys sent to Camp Green Lake have their own internal demons or weaknesses to confront: Zero is needy, insecure, and often seems surly from experiencing homelessness and having lost or been abandoned by his mother, Stanley has to learn to be less naive and develop a greater sense of self-confidence, and X-Ray is hampered in developing friendships by his tendency to be so transactional.
Cruelty or man's inhumanity to man is another important aspect of this novel. The boys at the camp have to learn to deal with the cruelty of the guards. The guards poured the boys' meager supply of water on the ground in front of them because they displeased one of the guards, which creates in the boys an "us versus them" mentality against their captors. Mr. Pendanski, for example, provokes Zero to such an extent that Zero hits him in the face with a shovel. Individual hostile authority figures present a constant threat to the boys.
Societal injustice is a central theme of this novel: the boys at the camp are victims of systemic injustice, being caught in a correctional system that routinely treats them with harshness. Stanley didn't even commit the crime for which he was sent to the camp. Societal greed conflicts with the boys' needs; the boys are subjected to digging holes in the hot desert by people in power who have internalized the larger society's valuing of material gain over the well-being of other, more vulnerable humans. Further, systemic racism comes into conflict with love and causes the people in the town of Green Lake to treat Kate and Sam cruelly when they become an interracial couple.