The book develops some aspects of Eco's A Theory of Semiotics which came out in 1976.
In the first chapter Eco develops Nietzsche's argument that the truth is a poetically elaborated "mobile army of metaphors, metonymies and anthropomorphisms" that subsequently gel into knowledge, "illusions whose illusory nature has been forgotten", as the metaphors are reduced to schemata and concepts.
In chapter two, working with ideas derived from Charles Peirce and Immanuel Kant, Eco compares linguistic and perceptual meaning when confronted with the unencountered.
Chapter three explores the Aztec encounter with the horse in terms of Cognitive Type, the private mechanism which allows identification of an object, and the Nuclear Content, which clarifies the relevant features inter-subjectively. To this is added Molar Content, which provides a much broader range of knowledge, even if resticted to specific competences. From these he develops an understanding of social elements in the organisation of knowledge.
With chapter four he discusses the different ordering of knowledge with a dictionary and an encyclopedia - that is, the differences between categorical knowledge and knowledge by properties. Using the example of the arrival of the first platypus in Europe, Eco looks at the problem faced by scientists in their attempts to classify the creature for eighty years and the contractual nature of the negotiations which produce shared meaning.
In chapter five Eco discusses the Sarkiiapone, an animal whose sole nature is that it is fictive. He then dicusses how the meaning of a term is affected by the context using examples to tease out different meanings.
Chapter six deals with iconism and hypoicon. Eco compares and contrasts "likeness" and "similarity" in relation to perception and conception. Basic semiotic processes taking place within perception providing determinations from which cognitive types can be constructed, with all the cultural baggage that is involved.