The book explores Birnbaum's concepts of a 'principle' and 'pattern' in art, which he developed and wrote about in the 1920s and 1930s. The book also traces the roots of Birnbaum's theoretical work within the broader context of European art-historical and philosophical thinking and describes the impact his concept of 'development' in art had on the next generation of art historians, who became formative figures in the evolution of this field.