The theme of appearance versus reality is present throughout Shutter Island. The novel follows Teddy Daniels, a US Marshall who tries to find a missing patient, Rachel Solando. As the novel continues, he finds evidence to suggest that psychotropic drugs are being tested on criminals and that patients are being mistreated. However, in the climax of the novel, it is revealed that his version of events is false, and he is in fact himself a patient in the asylum.
The author uses narrative voice and point of view to question the reader's perception of events. As Teddy is the first-person narrator of the text, the reader gains all their information through him and instinctively trusts his version of events. Additionally, Teddy is presented as an identifiable and honorable war hero, which invites the reader to trust him further. When it is revealed that his interpretation of events is not accurate, the reader therefore questions their own perception of the text and how readily they trusted Teddy's account.
In a philosophical sense, the author is also inviting us to question our own perception of reality. The novel warns that often others dictate what is real and what isn't to us. The idea that institutions have the power to determine reality is explored in the novel. At the asylum, psychiatrists and doctors determine who is insane and who is not. Consider the following quote:
If you are deemed insane, then all actions that would otherwise prove you are not do, in actuality, fall into the framework of an insane person's actions. Your sound protests constitute denial. Your valid fears are deemed paranoia. Your survival instincts are labeled defense mechanisms. It's a no-win situation.