Johann Nikolaus Forkel's metaphysical essay on the philosophy, aesthetics, and rhetoric of music forms the Einleitung to volume one of the Allgemeine Geschichte der Musik (1788). Forkel shows in this writing that the musical expression of man stands in synchronic relationship with man's development of mental and perceptual capacities, termed Empfindung. He presents his understanding of tonal structure and its gradual development in various cultures. These ideas, new in his time, are based on precepts of universal history with the purpose of loosening the philosophical orientation of music from its mooring in mathematics and placing it within a linguistic model. Given this change of conception, he could include musical lectures and concerts in the fine arts offerings at the University of Göttingen. Evidence is presented for Forkel's strong reliance on writings by Johann Adolph Scheibe as the starting point of his essay.
Forkel describes how each musical element contributes to the expression of the flow of feelings in a composition, because, as he sees it, music is a full-fledged language of feelings. Musical rhetoric provides the structure through which to channel the expression of feelings in a composition, which consists of several aspects of a primary feeling. The narrative of feelings throughout an entire work constitutes a process of experiencing feelings, a sequence better understood in the twentieth century than in the eighteenth.
Forkel balances four dichotomies present in eighteenth-century thought: interpretive versus descriptive history, music as language versus mathematics, harmony versus melody, and instrumental versus vocal music. Some of these dichotomies possibly result from the physical manner in which we process perceptions and information in the brain. Under contemporary rubrics of cognitive psychology, developmental psychology, linguistics, biomusicology (perception), aesthetics, and rhetoric, music theorists in the twentieth century are pursuing the same complex of interwoven topics as did Forkel. In addition, Forkel's essay suggests to performers ways of consciously building artistic conceptions of eighteenth-century compositions. As a musical philosopher, Forkel not only summarizes some significant intellectual trends of the eighteenth century, but treats issues that are of interest to twentieth-century music theory, philosophy, and performance.