In Ignes Fatui, Charles Thomas Taylor examines various misapprehensions and misconceptions that interfere with clear rational thought. The title of the book, a loanword from Medieval Latin, means "foolish fires"—lights that occasionally appear in the nighttime over marshy ground and are frequently attributable to the combination of gases emitted by decomposed organic matter. The term is sometimes used in modern times to suggest deceptive thoughts, goals, or hopes. The first four chapters of Ignes Fatui consider various common illusions that interfere with sound objective thought in society. The fifth and final chapter considers the illusions that prevent and consequently forestall any form of effectual subjective thinking in personal life. The primary objective of the book is to attempt to improve rational thought and thereby reverse the general decline of faith in the power of reason today.