Vygotsky discussed how early thinking in infants developed independently of speech, and speech could serve emotional and social functions independently of thinking in the child, “less than one year old.”
“But the most important discovery is that at a certain moment at about the age of two the curves of development of thought and speech, till then separate, meet and join to initiate a new form of behavior. Stern’s account of this momentous event was the first and best. He showed how the will to conquer language follows the first dim realization of the purpose of speech, when the child ‘makes the greatest discovery of his life,‘ that ‘each thing has its name‘ [40, p. 108].”
“This crucial instant, when speech begins to serve intellect, and thoughts begin to be spoken, is indicated by two unmistakable objective symptoms: (1) the child’s sudden, active curiosity about words, his question about every new thing, ‘What is this?’ and (2) the resulting, rapid, saccadic increases in his vocabulary.” (Vygotsky, Thought and Language, 1934, translation 1962)