Aristotle argued that to understand the essence of a thing, it is necessary to understand its distinctive function. For example, one cannot understand what it is to be a musician unless one can understand that the distinctive function of a musician is "to play on a musical instrument with a certain degree of skill." Whereas human beings need nourishment like plants and have sentience like animals, their distinctive function, says Aristotle, is their unique capacity to reason. Thus, the Supreme Good or Happiness, for human beings is to lead a life that enables them to exercise and to develop their reason, and that is in accordance with rational principles.