At its core, Wieland is an epistemological investigation that examines how we know the world. The novel was written about a century and a half after René Descartes published his major works on philosophy, specifically a branch of philosophy called skepticism.

Descartes understood that the answer to the question “how do we know the world?” is through our senses. We know that fire is hot and honey is sweet because we have experienced them as such, and with enough experiences we construct a view of the world solid enough to help us navigate it. However, our senses can and do deceive us. Optical illusions that distort size and distance or imagining our name being called when no one is calling are a couple examples of how our senses are susceptible to manipulation and error.
Not only that, but Descartes acknowledged that many of the beliefs he once had have turned out to be completely false. Whether it’s the geocentric model or the existence of Santa, beliefs that were once held so strongly that any doubt of them would be absurd can be discredited so powerfully that considering even a hint of truth in them becomes the new absurd. For Descartes, this meant that nothing can be taken for granted. If you were wrong once about something you can very well be wrong about everything and therefore, must be skeptical of everything.
The other central idea to Descartes’ claims is our experience of dreams, which he writes about extensively. His argument regarding dreams can be summarized in the following quote:
There are no conclusive signs by means of which one can distinguish clearly between being awake and being asleep, and that I am quite astonished by it; and my astonishment is such that it is almost capable of persuading me that I am asleep now.
Descartes, Rene. Discourse on Method and the Meditations. 1637
In other words, most of us, Wieland’s Clara included, have had dreams that vividly mimic reality and real moments that feel excerpted from dreams. By questioning the boundaries that demarcate dreams from reality as a whole, we participate in what he calls global doubt, a type of doubt we cannot step out of because it engulfs our very thought processes.
This is where he introduces the idea I believe is most central to Wieland: the Evil Genius. In his skeptical approach to epistemology, Descartes proposed the existence of an evil genius/demon, whose sole intent is to deceive humanity so convincingly his deceptions become indistinguishable from reality. There is nothing you can do to prove you are actually on your computer right now and not being efficiently deceived into thinking you are. For Descartes, the evil demon is not a literal entity, but because the possibility of his existence cannot be disproved, we are naturally deprived from truly knowing anything about the world.
Cartesian themes in Wieland are nearly impossibly to ignore, and it is probable that Brown would have been aware of the discourse at the time of writing this novel. From the shifts in beliefs throughout the novel to recurring dreams that merge with reality and the involvement of an evil genius who masterfully deceives the characters, this novel reads like an exemplary tale of radical skepticism.