Mental progress is largely dependent on the transfer of words to newer and larger spheres of experience, the modification of old words or the formation of new ones to express the increasing complexity of relations men discover to exist between things. In the instances already cited some of the transferred words lost their more general meaning and became specialized, as in the case of "meat," "spill," etc. Other words, like "head," though they may keep their specific objective meaning, may come to be used in a generalized intellectual sense. One of the chief ways by which a language remains adequate to the demands of increasing knowledge and experience of the group is through the transfer of words having originally a purely objective sense to emotional and intellectual situations. These words, like "bitter," "sour," "sharp," referring originally only to immediate physical experiences, to objects perceived through the senses, come to have intellectual and emotional significance, as when we speak of a "sour" face, a "bitter" disappointment, a "sharp" struggle. Most of our words that now have abstract emotional or intellectual connotations were once words referring exclusively to purely sensible (sense perceptual) experiences. "Anxiety" once meant literally a "narrow place," just as when we speak of some one having "a close shave." To "refute" once meant literally "to knock out" an argument. To "understand" meant "to stand in the midst of." To "confer" meant "to bring together." Sensation words themselves were once still more concrete in their meaning. "Violet" and "orange" are obviously taken as color names from the specific objects to which they still refer. Language has well been described as "a book of faded metaphors." The history of language has been to a large extent the assimilation and habitual mechanical use of words that were, when first used, strikingly figurative.