The science, that is, the general and universal properties, of language. Interest in language seems to have arisen in the earliest history of the human species. Every culture has left records that reveal philosophical or practical concerns for this unique human characteristic, and these concerns have varied over time. The middle of the twentieth century saw a shift in the principal direction of linguistic inquiry from one of data collection and classification to the formulation of a theory of generative grammar, which focuses on the biological basis for the acquisition and use of human language and the universal principles that constrain the class of all languages.