'Know Thyself' advised the ancient Greek sages who defined us as rational animals. Rationality, they thought, could be achieved by controlling the emotions and avoiding logical fallacies. We now know that the brain is a great deceiver. Unconscious biases drive us to believe and do things that the conscious mind explains in self-serving stories, making us appear more rational to ourselves than we really are. Modern science has taught us that rationality involves much more than just controlling the emotions and avoiding fallacies. Today's rational animal--what we call the critical thinker--must understand the unconscious biases that are directing many of our most important judgments and decisions. The Critical Thinker's Dictionary explores the insights of ancient and modern philosophers along with the latest findings in such fields as neuroscience and behavioral economics to lay out the many obstacles and snares that await anyone committed to a rational life.