The basic impoverishment of deafness is not lack of hearing
but lack of language. To illustrate this, we have only to
compare a 4-year-old hearing child, with a working vocabulary
of between 2,000 and 3,000 words, to a child of the same
age, profoundly deaf since infancy, who may have only a few
words at his command. Even more important than vocabulary
level, however, is the child’s ability to use his language
for expressing ideas, needs, and feelings. By the age of 4
years, the hearing child in all cultures has already grasped
the rules of grammar syntax that enable him or her to combine
words in meaningful ways.