The third main way in which the vocabulary develops is by changes in the meaning of words. Here we are not concerned with new words, as in the last two chapters, but with existing words which come to acquire new meanings. This kind of development is very common, for nearly every word has changed its meaning to some extent in the course of its history. It is also extremely varied and elusive, for changes take place continually, and not in the readily observable realm of spoken sound and written symbol, but in the realm of the mind; and they are affected by all the complexities of human psychology and the life of society. All we can do in this chapter is to describe the most common kinds of change and indicate some of their more important causes. The categories we use will cover many, perhaps most, cases; but some individual words will still fall outside their scope.