Denial
Denial is the most prevalent theme in A Monster Calls. Throughout the novel, Conor is in such a deep state of denial over his mother's terminal illness that he cannot—even in his thoughts—use the words "cancer" or "death." Although Conor later admits that he knows all along that she will not survive her cancer, he insists on telling himself and others that her treatments will make her better; meanwhile, he watches her condition worsen. Ness depicts Conor's denial as an instinctive response to the incomprehensible nature of death and as a fundamental stage of the mourning process. Ultimately, Conor can only let go of his mother once he faces the truth of his desire to end the pain of watching her die.
Acceptance
The theme of acceptance functions as a contrast to the theme of denial. As long as Conor is unwilling to accept the unfair reality of his life as the young son of a mother with terminal cancer, he remains in a chaotic state of isolation. However, once Conor later in the novel admits that he has always known his mother would die, it becomes clear that some part of him has already accepted her death, while another part of him denies this reality. A nuanced understanding of the stages of grief sees acceptance of a loved one's death not as the last stage in a sequence but as a state of grieving that a person may return to and depart from repeatedly. In this way, Ness depicts acceptance as something Conor constantly grapples with before he finally feels the relief that true acceptance grants.
Anger
As another stage of grief, anger plays a crucial thematic role in A Monster Calls. To remain isolated in his grief, Conor instinctively lashes out in anger at the people who attempt to express care toward him. His anger alienates him from his friend Lily, his father, his grandmother, his teachers, and even the monster who has come to help heal him. While there is something satisfying about anger, Conor often feels ashamed after an outburst. But his anger takes possession of his mind, and Conor acts out before considering the consequences of his actions, such as when he twists the mechanism of his grandmother's antique clock. Conor's mother tries to teach Conor that his anger is warranted, and the monster helps Conor express his anger by destroying Grandma's sitting room. As the story progresses, it becomes clear that Conor is seeking punishment for his outbursts because he believes that his secret wish to end the pain of watching his mother die means he is worthy of punishment.
Truth
Throughout the novel, the monster insists to Conor that he will have to speak his truth after the monster tells its three tales. Conor refuses to admit there is any truth to reveal, but finally, after much coaxing, he is able to admit the shameful truth which has kept him isolated and afraid. Conor's truth—that he knows his mother is going to die and wishes for it to happen so that he can be relieved of the pain of watching her suffer—turns out to be not as shameful as he believes. While Conor fears the possibility of anyone knowing how he feels, as he believes he must appear to maintain hope that his mother's treatments will save her, the monster informs Conor that it is an understandably human emotion simultaneously to wish her to get better and to wish for her to die so he no longer has to endure the pain of waiting. Ultimately, Conor's ability to accept the truth of his mother's condition as well as his own feelings frees him from the burden of keeping his secret and allows him to be honest not just with himself, but with his mother in her dying moments.
Cognitive Dissonance
Another of the novel's major themes is cognitive dissonance—i.e. the state of holding thoughts or attitudes that seem to be inconsistent or directly contradictory. Much of Conor's distress stems from his mental and emotional capacity simultaneously to hope for his mother to get better and to realize that she will not get better, which leads him to wish that her death will come soon so he may find relief from the pain of living in uncertainty. As Conor's attitude oscillates between denial and acceptance, optimism and pessimism, he feels guilty for not being true to his belief that his mother's condition will improve. While Conor's ability to hold on to these seemingly contradictory attitudes generates shame for him, the monster assures Conor that it is a fundamental aspect of being a human to believe in two things at once.
Isolation
Another aspect of Conor's grieving process is isolation. After the teachers and pupils at school learn of Conor's mother's illness, Conor senses their behavior toward him suddenly change: their expressions become mournful, and they are self-conscious about showing happiness. Conor feels he is no longer accepted as he normally would be and so he grows increasingly isolated, eating alone at lunch and speaking to no one, as though he is invisible. Conor also feels isolated within his family, as his father lives abroad and he doesn't get along with his grandmother. Although Conor loves his mother and helps her when her treatments leave her fatigued, he is isolated from her too, as he cannot admit to her or himself that he believes she will not survive. The monster helps Conor move out of isolation by teaching him to accept the unfairness of his life and by standing by as Conor repairs his relationship with his grandmother and mother.
Healing
Healing is another of the major themes in A Monster Calls. Throughout the novel, Conor's mother undergoes chemotherapy treatments in an effort to survive cancer. Although Conor believes, deep down, that his mother will not survive, he upholds the narrative that her treatments are going to heal her. Conor learns from the monster that the yew is an ancient healing tree, and he insists that the monster has come to heal his mother. However, the yew tree monster eventually reveals that he has come to heal Conor, not his mother. While it is true that anti-cancer drugs can be made from yew trees, the monster's true purpose is to help Conor work through his conflicting emotions and heal the internal rift in his life by showing him how to accept the truth of his unfair situation.
