Vygotsky's studies in concept formation, based on the experiments he performed with Sakharov, are reported in chapter 5 of Thinking and Speech and they rank among his best-known work. In Vygotsky's theory, the development of concepts occupies centre stage and plays a role far beyond that of a particular kind of logical operation or set of mental functions. According to Vygotsky, word meanings are infused with, or constituted by, concepts without which they are limited to a primitive nominative function. He points out that word meaning is a phenomenon of both speech and thinking and that in psychological terms, word meaning is ‘nothing other than a generalization, that is, a concept’ (1987, p. 244). Crucial to the link between concepts and word meaning is the fact that they develop or, more precisely, the fact that word meanings change because concepts do not emerge fully formed with the acquisition of new words by the child but follow a long process of development through a complex series of stages and phases. Vygotsky regards this discovery as ‘our new and fundamental contribution to the theory of thinking and speech’ (p. 245). The concept of ‘word meaning’ is a pervasive theme in Vygotsky's work and he devotes the last chapter of Thinking and Speech, entitled ‘Thought and word’, to a discussion of word meaning, a phenomenon of both speech and thinking, and argues that it provides an appropriate unit or ‘microcosm of human consciousness’ (p. 285).