Offering an explanation of the fundamental nature of thought, this book posits the idea that thinking involves the processing of mental representations that take the form of sentences in a covert language encoded in the mind. The theory relies on traditional categories of psychology, including such notions as belief and desire. It also draws upon and thus inherits some of the problems of artificial intelligence which it attempts to answer, including what bestows meaning or content upon a thought and what distinguishes genuine from simulated thought.