Another psychologist who was influenced by Selz, although he deplored Selz's lack of attention to the relation of thought to speech, was Lev Semenovich Vygotsky (1934/1962, p. 122). Vygotsky formulated a theory of mental growth in which instruction guides development rather than following it. His use of a dynamic research methodology in which thinking processes are observed as they develop under the conditions of instruction has been adopted by. Western researchers (see Kantowski, 1979, pp. 132-133, for a description of the methodology). And his concept of the zone of proximal development the difference in level of difficulty between problems that one can solve alone and those one could solve with the help of others—is being used by researchers interested in the social mediation of cognitive change (Newman, Griffin, & Cole, 1989). Vygotsky was one of the developers of the Soviet theory of abilities, which was extended to the study of mathematical abilities by Vadim Andreevich Krutetskii (1968/1976). (See Goldberg, 1978, for a review of other relevant Soviet and East European psychological research.)