"The Man Who Was Thursday: A Nightmare" is a 1908 novel by G. K. Chesterton that blends philosophical inquiry with a metaphysical thriller narrative

. The story follows Gabriel Syme, a poet and police detective who infiltrates a secret anarchist council in London, whose members use days of the week as codenames. Syme assumes the identity of "Thursday" after the previous holder of the title dies, and he joins six other council members named Monday through Sunday, with Sunday as their enigmatic leader

. The novel explores themes of order versus chaos, identity, morality, and authority through Syme's interactions with the anarchists, many of whom turn out to be undercover detectives themselves. The anarchist council plans a political assassination, which Syme aims to thwart. The story unfolds as a surreal and suspenseful chase, filled with deception, double-crossing, and philosophical dialogue about the nature of belief and human behavior

. Chesterton's work is noted for its allegorical depth and satirical critique of nihilism and anarchism prevalent in early 20th-century thought. It remains a classic of crime fiction and a fascinating mystery that blurs the lines between freedom and order, fact and fiction

. In summary, "The Man Who Was Thursday" is a thrilling and thought-provoking novel about a detective's infiltration of an anarchist group, exploring profound questions about society and human nature through a gripping and often surreal narrative