Results of studies on communication discrepancy and persuasion effect have been quite confusing. Such theories as the cognitive dissonance theory, social judgment theory, and psychological reactance theory are often applied to explain the findings ; however, explanations based on these theories have not been convincing, for these theories themselves often lack the firm ground to support their findings. In this paper, the Yin and Yang Theory of Cognition will be presented with the intent of opening up a new perspective to the series of studies made so far on the current subject, including those by the writer. The basic principle of the Yin and Yang Theory, which has its origin in an ancient Chinese philosophy, is that all matters are comprised of two opposite, but complementary, components, yin and yang, commonly translated as dark and light. Human cognition is not an exception to this principle, and it is the contention herein that, for every manifest yang (or yin) which the human mind expresses as its opinion, there is always its latent counterpart opinion yin (or yang) which remains unexpressed. What is involved in persuasive communication is the activation process of the latent opition into the manifest realm. Through this process an individual sharing the same opinion as another may take on an opposite opinion or an individual with an opposite opinion may be persuaded to change his opinion. The theory has resemblance to the ancient Chinese philosophy of Q_i, and the present paper will discuss five basic principles of the philosophy, with discussions on approximately thirty postulations to follow in subsequent papers.