Studies of copular forms are extremely relevant to issues in philosophy, psychology, and linguistics. Psychologists have recently argued that the most distinctive aspect of human language is its combinatorial nature (e.g., Gentner, 2003; Spelke, 2003). They argue that this linguistic component might be what separates human from animal cognition. As combinatorial elements, copular forms have no equal. However, as the quote from Bertrand Russell implies, copular forms are multi-dimensional and complex. A full understanding of their use, linguistic variation, acquisition, and relation to cognition is a formidable task. While other papers in this special-issue volume of Bilingualism: Language and Cognition will address the different syntactic and semantic functions carried by these forms, in this commentary, I will focus on the relation between the acquisition of copular forms and cognitive development, or more specifically on the development of categorization.