In short, he shifted attention to the perception of different sounds. Boas begins by raising an empirical question: when people describe one sound in different ways, is it because they cannot perceive the difference, or might there be another reason? He immediately establishes that he is not concerned with cases involving perceptual deficit—the aural equivalent of color-blindness. He points out that the question of people who describe one sound in different ways is comparable to that of people who describe different sounds in one way. This is crucial for research in descriptive linguistics: when studying a new language, how are we to note the pronunciation of different words? (in this point, Boas anticipates and lays the groundwork for the distinction between phonemics and phonetics.) People may pronounce a word in a variety of ways and still recognize that they are using the same word. The issue, then, is not "that such sensations are not recognized in their individuality" (in other words, people recognize differences in pronunciations); rather, it is that sounds "are classified according to their similarity" (in other words, that people classify a variety of perceived sounds into one category). A comparable visual example would involve words for colors. The English word "green" can be used to refer to a variety of shades, hues, and tints. But there are some languages that have no word for "green".[65] In such cases, people might classify what we would call "green" as either "yellow" or "blue". This is not an example of color-blindness—people can perceive differences in color, but they categorize similar colors in a different way than English speakers.