This book is about rational thinking, although such a thing may not even exist. There are many signs indicating that basically, human thinking is not rational; not even when the methods of pure logic could be applied optimally in solving certain problems. I treated this matter in my book Ways of thinking. The present book does not rely on Ways of thinking, but complements it: We are going to discuss the topic from the aspect of the world revealed by John von Neumann's theory of games. The book is divided into three parts. In the first part, the basic concepts of game theory are introduced, together with several games and life situations which illustrate the theory's ideas and typical approach towards different psychological and moral issues. The second part is probably more difficult to read than the first or the third part. It will be demonstrated in five different fields of science (mathematics, psychology, evolutionary biology, economy and quantum physics) how different the forms in which the ideas of game theory can manifest themselves can be. In the third part, the lines converge. This part is purely about psychology, about rational thinking in particular, which is not at all the same as logical thinking. Key-words: human thinking, theory of games, cognitive sciences.