Content Warning: This section includes references to graphic descriptions of violence against children and the commercial sexual exploitation of children. Furthermore, because the novel is set in 1896, it includes dialogue that reflects the language of that era.

The psychological debate between free will and determinism revolves around one fundamental question: Are human beings free to choose their courses of action? Proponents of free will argue in the affirmative, while advocates of determinism insist that human behavior is a product of factors more complex than mere choice. The American philosopher William James breathed new life into this old debate in 1884, at Harvard Divinity School, with a lecture titled “The Dilemma of Determinism.” In the lecture, James argues against determinism as leading into an amoral abyss in which the powerful can justify any cruelty by claiming that it could not have been otherwise—an argument that later seemed to anticipate the ideologically driven horrors of the 20th century. In The Alienist, Dr. Kreizler advances a semi-deterministic view of behavior that emphasizes habits and assumptions forged in childhood, often through trauma. This view guides Kreizler and his team of investigators as they hunt for a serial killer, but it also brings them into direct conflict with powerful people who regard determinism as a threat to the established social order.