The book is divided into three parts and explores a wide range of topics, including consciousness, the nature of reality, time, space, and the concept of higher dimensions. The title "Tertium Organum" is Latin for "the third canon of thought," and refers to the idea that there is a higher level of thinking beyond the first two "organs" of the mind, which Ouspensky identifies as the senses and reason. The book had a significant influence on many thinkers of the 20th century, including Aldous Huxley and George Gurdjieff.