Mental Illness
The central theme of the novel is mental illness, specifically illness that involves psychopathy and the criminally insane. Each of the characters is either a psychiatrist or a patient, but even the doctors seem to suffer from some kind of psychological disorder, such as Dr. Cawley, whose belief in his theory that all people are violent at their core borders on obsessive. Teddy, the main character, is said to be delusional, and the patients have varying degrees of psychopathy—for example, Vincent Gryce, who scalped this family and wore the tops of their heads as hats. There is also an assertion in the book that a person can be driven insane given the right combination of drugs and circumstances.
Conspiracy Theories
Another theme that is revisited many times throughout the novel is conspiracy theories. George Noyes, a prisoner, had talked about illicit experiments and abusive treatment of patients at Shutter Island, and Teddy bought into these. It is apparent that it would be very easy for the authorities to cover up any illicit activity by claiming the accuser was delusional or paranoid. The novel takes place in a time of acute paranoia: worry about a Japanese "Fifth Column" within the United States, about Communist infiltrators, Nazi scientists working under assumed identities in the United States, and CIA experimentation. By filling his novel with conspiracy theories that did, in fact, turn out to be true (the latter two, specifically), Lehane intensifies the suspicious atmosphere on Shutter Island.
Grief
Many of the characters in Shutter Island suffer from grief over lost loved ones. Teddy, most obviously, suffers from the memory of his wife Dolores, while Chuck suffers angrily from the memory of his girlfriend's internment. Dr. Cawley mentions the loss of a lover in Paris, who, amid all of the carnage during the war, simply tripped and fell and died. Rachel Solando has been driven mad by the grief of her husband's death, and grief for the loss of her children whom she killed. If we read the ending of the novel to mean that Teddy is, in fact, Andrew Laeddis, then the novel is no longer a mystery but a meditation on grief, and the question of whether it is possible to let go of painful memories, or to forgive oneself for wrong-doing. Teddy's decision to have himself lobotomized suggests that, for him at least, it is impossible—particularly in a world as harsh as the one he has experienced.
Masculinity
The novel opens with Teddy's reflection on being a "bad sailor"—the fact that, unlike his father, he can't handle the wide open sea. He is unsuited to the harshness and unfathomability of the ocean, and, more broadly, of the world. Throughout the novel, Teddy is presented as falling short of several masculine ideals: he wears a feminine, floral-print tie, he doesn't drink, he suffers from seasickness. Teddy is unable to maintain the rugged, masculine ideal of the detective: hard-boiled emotionlessness. Every act of violence— and having to suppress his emotion at an act of violence, as a man is expected to do—causes Teddy significant pain. Other male characters, too, like Chuck/Dr. Sheehan and Dr. Cawley, consider themselves out of place because of their depth of feeling, which damages their career in hyper-masculine professions like the U.S. Marshals and psychiatry.
Violence
Shutter Island contains numerous scenes of violence, especially towards the end, when Teddy is fleeing the guards. But violence, and its human aftermath, suffuses all of the characters' interactions. Dr. Naehring diagnoses Teddy as a "man of violence," while the warden openly observes that society is a thin veneer for the openly violent character of man and nature. The violence that haunts Teddy most is the violence of the war, which most Americans remember heroically. He draws no satisfaction and no moral closure from the murder of the guards at Dachau, for example, and he still feels guilt and anguish at having made intelligence mistakes that led to the deaths of his platoon, or simply from seeing friends be killed. Those memories have made Teddy a skilled marshal, adept at thinking his way into the minds of violent criminals, but that has taken an enormous personal toll on him. If we read the novel to mean that Teddy has been lobotomized at the end, then this violence is undoubtedly the part of his mind that Teddy wants removed.
Truth and Power
On the novel's third day, Teddy's encounter with Dr. Rachel Solando introduces the idea that that institutional power dictates truth. Because the psychiatrists at Shutter Island are in the position of determining who is sane and who is not, they essentially have control over reality, by declaring those who disagree with them, or seek to criticize them, as insane. Lehane repeatedly draws parallels between the medical realities of the psychiatric profession and the profession's political abuses, as for example by Senator Joseph McCarthy, who accused his political opponents of being Communists. Teddy's struggle to establish what is really going on at Ashecliffe ultimately fails, and the reader is left without the sense that there is a firm truth beyond the reach of Ashecliffe's doctors.
Nature
Shutter Island is filled with vivid natural imagery, like the terror of the storm, the overwhelming ocean, the jagged cliffs, the portentous sky before the rain. The natural world in Shutter Island is one that is hostile to human beings, not simply because of its violence, but because if its unfathomable enormousness. Teddy's first memory is of vomiting after seeing the ocean's enormity, and the many overwhelming natural occurrences on the island drive home the sense that the natural world is one that is fundamentally inhospitable to human beings and that ultimately exceeds our comprehension. The warden surmises that nature is simply violence, and that all attempts to establish a more civilized society are doomed to fail, because, as natural beings, humans' violent tendencies cannot be suppressed.
