The words discussed so far have been limited to one type: those whose meaning identifies the members of a class. For example, the word chair is used correctly when it is applied to the class which includes objects as different as straight chairs, folding chairs and rocking chairs. The same skill in identifying instances of the same class is required for understanding some types of verbs. For example, all people walk differently, but native speakers of English use the word walk correctly when they realize that these minor differences are irrelevant. But not all words in a language involve the identification of classes. In fact, the mastery of a working vocabulary in any human language appeals to a wide range of intellectual skills, some easier and some more difficult than those required for grasping the meaning of common nouns and verbs. As an example of a relatively easy concept, consider what is required for understanding proper nouns: one must simply point out a single individual and attach a label, like John or Daddy. Because it is easier to associate a label with a single individual than to name a class with common properties, children master proper nouns first, sometimes when they are as young as six to nine months old. In contrast, a relational term like large or small constitutes a relatively complex concept. The correct use of words like these requires that two things be kept in mind: the absolute size of the object in question, and its position on a scale of similar objects. For example, an elephant which is six feet tall at the shoulders may be small as far as elephants go, but a dog of the same height would be huge. Five-and six-year-old children are unable to make the shift in perspective necessary for using relational words appropriately. In one well known experiment which documents this conclusion, children were engaged in a pretend tea party with dolls and an adult observer. The adult gave the child an ordinary juice glass and asked the child if it was large or small. Though all of the children in the study agreed that the glass was small from their own perspective, it appeared ridiculously large when placed on the toy table around which the dolls were seated. Nevertheless, the youngest children were still inclined to say that the glass was small when asked about its size with respect to its new context. Another complex concept underlies deictic expressions, which are words used to point to objects and indicate their distance from the speaker. For example, the speaker may use here or this to point out objects which may be close to him, while there and that are appropriate only when the objects are relatively far away. But since there are no absolute distances involved in the correct use of a deictic expression, children have difficulty determining when the "close" terms are to be preferred over the "far" terms. As with relational terms, it is necessary to take into account the size of the object pointed to. Thus a thirty-story building six feet in front of us is close enough to be called this building, but an ant removed from us by the same distance is far enough away to be called that ant. Common and proper nouns, relational terms and deictic expressions do not exhaust the range of concepts mastered by children, but they do illustrate the variety of tasks involved in acquiring the vocabulary of a first language. Linguists can examine the evidence from the acquisition of word meaning and find support for two fundamental hypotheses: that some concepts are more complex than others, and that the acquisition of language requires a considerable exercise of intelligence. This passage is probably an excerpt from an article on ______.