The theory of knowledge is the most important part-in fact, the very foundation of the Nyaya system. This book is an attempt to give a complete account of the Nyaya theory of knowledge. It is a study of the Nyaya theory of knowledge in comparison with the rival theories of other systems, Indian and Western and a critical estimation of its worth. Though theories of knowledge of the Vedanta and other schools have been partially studied in this way by some, there has as yet been no such systematic, critical and comparative treatment of the Nyaya epistemology. The importance of such a study of Indian realistic theories of knowledge can scarcely be overrated in this modern age of Realism. The scope of the book is limited to the history of the Nyaya philosophy beginning with the Nyaya-Sutra of Gautama and ending with the syncretic works of Annam Bhatta, Visvanatha and others. It does not, however, concern itself directly with the historical development of the Nyaya. Nyaya as an art or science of reasoning existed in India long before the time of Gautama, the author of the Nyaya-Sutra. As a matter of fact, it has been admitted by Vatsyayana, Uddyotakara, Jayanta Bhatta and others that Gautama was not so much the founder of the Nyaya as its chief exponent who first gave an elaborate and systematic account of an already existing branch of knowledge, called nyaya, in the form of sutras or aphorisms. It is in these sutras that the Nyaya was developed into a realistic philosophy on a logical basis. What was so long mere logic or an art of debate became a theory of the knowledge of reality. It is for this reason that the present work is based on the Nyaya-Sutra and its main commentaries.