This book introduces the reader to dialectical reasoning, a kind of thinking found in ancient East Asia and Greece as well as modern Europe and elsewhere in the contemporary world. Here, we focus on Zeno, Socrates, Kant, and Marx, with appendices on Karl Popper and the Frankfurt School. Studying these sources from different regions and periods, we see that the thinking is essentially the same. A similar formal pattern recurs: dialectical thought is always oppositional and self-relational.This book is written in the belief that dialectical thought is understandable and relevant to many kinds of persons. One does need to have a degree in philosophy to be moved by the great dialecticians. One may even be a dialectician without academic training.