The total number of possible combinations of phonetic elements is also unlimited, but only a limited number are used to express ideas. This implies that the total number of ideas that are expressed by distinct phonetic groups is limited in number. We will call these phonetic groups “word-stems.” Since the total range of personal experience which language serves to express is infinitely varied, and its whole scope must be expressed by a limited number of word-stems, it is obvious that an extended classification of experiences must underlie all articulate speech. This coincides with a fundamental trait of human thought. In our actual experience no two sense-impressions or emotional states are identical. Nevertheless we classify them, according to their similarities, in wider or narrower groups, the limits of which may be determined from a variety of points of view. Notwithstanding their individual differences, we recognize in our experiences common elements, and consider them as related or even as the same, provided a sufficient number of characteristic traits belong to them in common. Thus the limitation of the number of phonetic groups expressing distinct ideas is an expression of the psychological fact that many different individual experiences appear to us as representatives of the same category of thought. As an instance we may mention the color terms of different languages. Although the number of shades of color that may be distinguished is very great, only a small number are designated by special terms. The number of these terms has considerably increased in recent times. In many primitive languages the groupings of yellow, green, and blue do not agree with ours. Often yellow and the yellowish-greens are combined in one group; green and blue, in another. The typical feature which occurs everywhere is the use of one term for a large group of similar sensations.