According to Lev S. Vygotsky (1896-1934), the highest levels of abstract thinking and self-regulation in preschool development are established in "pretend play using object substitutions." An extensive research literature supports Vygotsky's empirical model of the internalization of self-guiding speech (social speech > private speech > inner speech). Vygotsky also introduced an entirely new way of assessing the child's ability to learn by using the assistance of a more knowledgeable person -- "the zone of proximal development." These concepts are fundamental to Vygotsky's theory of how consciously directed mental functions develop as neurological systems. A chronology is provided to clarify the out-of-sequence and often piecemeal publication of his writings, revealing their theoretical cohesiveness and integrity.