This paper describes a computer program, called Copycat, that models how people make analogies. It might seem odd to include such a topic in a collection of papers mostly on the immune system. However, the immune system is one of many systems in nature in which a very large collection of relatively simple agents, operating with no central control and limited communication among themselves, collectively produce highly complex, coordinated, and adaptive behavior. Other such systems include the brain, colonies of social insects, economies, and ecologies. The general study of how such emergent adaptive behavior comes about has been called the study of "complex adaptive systems". <p> The Copycat program is meant to model human cognition, and its major contribution is to show how a central aspect of cognition can be modeled as the kind of decentralized, distributed complex system described above. In doing so it proposes principles that I believe are common to all complex adaptive systems, and that are particularly relevant to the study of immunology. <p> Copycat was developed by Douglas Hofstadter and myself, and has been described previously in [3, 10, 11, 16, 17]. This paper summarizes these earlier works, and makes explicit links to the immune system.
