Turtles All the Way Down is primarily a novel of ideas. Some of the themes it explores are mental health and illness, love, friendship, agency, death, pain, and the power of language. These themes are woven together with considerable complexity, one of the most interesting instances being the relationship between language and experience. Green often talks about the inadequacy of language to express certain vital aspects of human existence. Aza Holmes, the protagonist, suffers from OCD and anxiety, and Dr. Singh, Aza's therapist, tells her that "pain is the opposite of language" and can only be described in metaphors. She continues:
And we're such language-based creatures that to some extent we cannot know what we cannot name. And so we assume it isn't real. We refer to it with catch-all terms, like crazy or chronic pain, terms that both ostracize and minimize. The term chronic pain captures nothing of the grinding, constant, ceaseless, inescapable hurt. And the term crazy arrives at us with none of the terror and worry you live with. Nor do either of those terms connote the courage people in such pains exemplify, which is why I'd ask you to frame your mental health around a word other than crazy.
Dr. Singh points out that if you want to talk about love, you have the poetry of Shakespeare and Keats to draw upon, but elsewhere in the text, Green shows that love is not necessarily any easier to describe than pain or anxiety, despite being the subject of so much poetry. Love is described as "both how you become a person and why" and as finding "someone who sees the same world you see." Later in the novel, Aza notes a linguistic peculiarity of the way people talk about love:
It's a weird phrase in English, in love, like it's a sea you drown in or a town you live in. You don't get to be in anything else—in friendship or in anger or in hope. All you can be in is love.
This anomaly does not reflect Aza's experiences particularly well, since other states of being can be equally immersive and intense (though she does not note that one can, of course, also be "in pain"). All novels describe experience using language; there is arguably nothing else to describe and nothing else with which to describe it. Few, however, examine the relationship between the two with the persistence of Turtles All the Way Down.