The Illusion of Reality was conceived during my tenure as director of the newly established Division of Information Science and Technology at the National Science Foundation in 1979-1981 as a partial response to the need for a textbook for students, both in and out of government, that would provide a comprehensive view of information science as a fundamental constitu­ ent of other more established disciplines with a unity and coherence distinct from computer science, cognitive science, and library science although it is related to all of them. Driven by the advances of information technology, the perception of information science has progressed rapidly: today it seems well understood that information processing biological organisms and information processing electronic machines have something basic in common that may subsume the theory of computation, as well as fundamental parts of physics. This book is primarily intended as a text for an advanced undergraduate or a graduate introduction to information science. The multidisciplinary nature of the subject has naturally led to the inclusion of a considerable amount of background material in various fields. The reader is likely to fmd the treatment relatively oversimplified in fields with which he is familiar and, perhaps, somewhat heavier sailing in less familiar waters. The theme of common principles among seemingly unrelated applications provides the connective tissue for the diverse topics covered in the text and, I hope, justifies the variable level of presentation. Some of the material appears here for the first time.