OF all the inventions of the human race nothing compares in importance, as regards mental development, with language. In the development of each person also, nothing exercises a greater influence in molding and developing thought and feeling than his language environment. The vocabulary of a person represents in a condensed and symbolic form all that he has experienced and imagined. The breadth of his mental experience is indicated by the number of words that have for him a meaning, while the accuracy of his thinking is shown by the constancy and exactness of meaning with which he uses words. The study of vocabularies ought therefore to be an important branch of psychological investigation.
Studies have been made of the number of words used by great writers, and by children a few years old. The latter studies have shown that a child may not use words that are perfectly familiar to him for months merely because he has no occasion to use them, e. g., words frequently uttered in the summer or when in the country may never be used in the city or in the winter. Adults are familiar with many words that they have rarely, perhaps never, used. The difficulties in the way of counting accurately the number of words used by an adult or even by a child over three years of age are almost insurmountable.
When we attempt to estimate the number of words that have a meaning for an individual, the difficulties are less although the number of words is much greater. The writer long ago estimated the number of words in his own vocabulary by going carefully through an unabridged dictionary and counting the number of familiar words on every tenth page (see Science, O. S., Vol. XVIII., pp. 107-108). Since then he has often had his students estimate the number of concepts that they possessed by counting the number of words that had for them a fairly definite meaning, on a few pages of the dictionary, and then calculating from the proportion of familiar words the total number of words they knew.
When a student began, say on page 2, and counted all the words in bold-faced type and the number of these known on every fiftieth page, and then did the same beginning with page 20, the results were