The principal aim of this thesis is to argue that Humboldt's philosophy of language should be seen as a coherent system in which he envisages the interaction of thought, perception, and imagination. The thesis underlines the significance of Humboldt's approach to his philosophical sources and his clarification and development of ideas embodied in those sources. -- Humboldt starts with the basic assumption that the crucial characteristic of language is not only its structure - which is the mere prerequisite for its functioning - but also the speech-activity which creates and preserves language as a tradition. -- The discussion brings into focus the philosophy of such eminent philosophers as Leibniz, Condillac, Herder, and Kant, who not only exercised a profound influence on Humboldt's thought but also reoriented his way of thinking. -- A major argument centres on Herder's influence on Humboldt, and on the development of theories presented by the 'Ideologues'. The intellectual background of Humboldt's development of a general theory of human expression and activity played a major part in the fashioning of his linguistic philosophy, which was further supported by his study of anthropology, and leads to his final goal of 'Bildung'. -- Humboldt was the precursor of modern developments in comparative linguistic studies and ethnolinguistics, exploring the interrelationship of language and culture, to which he made lasting contributions in his studies of the Basque, Chinese, and Sanskrit languages. These he brought to the attention of scholars, and thereby initiated the objective study of these languages. -- It is also argued that Humboldt endeavoured to oppose the traditional concept of linguistics by an essentially new approach that reflects an ontological and phylogenetic interpretation of language.