It is widely acknowledged that natural language processing, as an indispensable means for information technology, requires the strong support of world knowledge as well as linguistic knowledge. This book is a theoretical exploration into the extra-linguistic knowledge needed for natural language processing and a panoramic description of HowNet as a case study. Readers will appreciate the uniqueness of the discussion on the definitions of the top-level classes HowNet specifies, such as things, parts, attributes, time, space, events and attribute-values, and the relations among them, and also the depth of the authors' philosophy behind HowNet.The book presents the attraction of HowNet's computability of meanings and describes how a software of the computation of meaning can collect so many relevant words and expressions and give a similiarity value between any two words or expressions.