This book is meant to help you improve your decision-making skills, by exploring how we 'know'. This results in a focus on beliefs. First, the nature of beliefs is clarified, by studying beliefs in their context of human brains and thinking. Second, the ways of obtaining beliefs are studied and the consequences for the truth and reliability of beliefs: observation, communication, conception, memorization and reasoning. Especially reasoning is studied into its details. We base our insights on observations of human reasoning as found in real life. The book serves both as a theoretical and a practical aid, with examples that will help you in making better judgments about the reliability of your beliefs. As a result, you will be able to recognize poor decisions and/or make better ones.