Considering practical problems which preoccupy psychologists and those working in the field of artificial intelligence, the author of this treatise offers an analysis of intelligent systems and a definition of the "intelligence" which characterizes their interaction with the environment. While stressing the diversity of the various kinds of systems, the author synthesizes ideas from the brain sciences, information technology, linguistics, medicine and psychology in order to present a central core of concepts that are common to all thinking about intelligent systems. The key issues developed are that of activity structures and protection structures, which are used to represent the adaptive behaviour of both natural and artificial systems and their relationships with the environment. The structure of the book has been organized so that readers without a knowledge of formal mathematics and logic can follow the main threads of the argument and extract those ideas that are relevant to their own concerns.