Over the centuries, the emotions have proven to be a notoriously recalcitrant philosophical subject, defying easy classification and stubbornly straddling accepted philosophical distinctions. With changing conceptions of the mind and its powers, categories such as emotion, desire, appetite, passion, feeling, and sentiment come and go. The general term the emotions is a relatively recent arrival to the English language, first gaining prominence in the nineteenth century, long after terms such as fear, shame, and joy were in common use. Its introduction was an attempt to clump together states that were supposedly marked by a degree of "emotion," a metaphorical extension of the original sense of the word, namely, agitated motion, or turbulence. Only the vagueness of the metaphor allows it to stretch far enough to cover typically quiescent "emotions" such as being pleased or sad about something.

Probably one influence on the extension of the term is the...