Much of current cognitive science is a debate between two views of thinking -- thinking as governed by mental rules and thinking as governed by similarity among ideas. Contributors to this volume explore these contrasting views in research on reasoning and concepts, and consider their merits from the perspectives of cognition, development, neuroscience, and artificial intelligence. The book evaluates the potential of each view to describe human cognition and examines whether systems compatible with these different perspectives might work together in explaining thought. While maintaining a high level of scientific sophistication, the book remains accessible to undergraduates and researchers in other fields.