What are the elementary building blocks of language? Which categories can they be assigned to, based on which criteria? What function do parts of speech or lexical categories have for the speakers of a language? This study provides answers to these theoretical questions, showing on the example of German that lexical categorization is dependent on cognitive and functional conditions ― not as a static structure, but rather as a dynamic process. The empirical part of the study shows that this has consequences, especially for writers of German.