Although F.A. von Hayek is well-known as a Nobel Laureate in economics, in his youth he considered a career as a psychologist. His interest in psychology continued and led to this book, which has been translated from the English and is being published in German for the first time as a part of his Collected Works. This is an inquiry into the foundations of theoretical psychology. It is Hayek's opinion that sensory perception must be regarded as an act of classification. We do not perceive the unique qualities of individual objects, but always qualities which the objects have in common with other objects. Hayek's analysis of the sensory order and the relationship between sensory and physical phenomena should be of interest not only to psychologists but also to philosophers, neurophysiologists, researchers who study the brain and others. unrevised e-book edition 2023; original edition 2006