This chapter acquaints the reader with the circle of ideas about the human thinking and its laws. If we know something in general about the world, then this is due to the fact that we know how to reason. While the logic or science about how to think correctly and how to come to true conclusions was developed in the course of millenniums, the science of knowledge as the result of reasoning arose in our time within the framework of a scientific direction called ‘artificial intelligence’. The use of computers in all spheres of our life requires studying and a fundamental understanding of how men extract knowledge from observations. Plausible reasoning, «fuzzy» reasoning reflecting the properties of human thinking in everyday life, became the subject of study in computer sciences, cognitive psychology, logic, and mathematics. The problem of the synthesis of knowledge and reasoning is not yet solved, but it is the forward edge of studies, and the experience of past times is extremely valuable for these studies.