But the exercise of labor power, labor, is the worker’s own life activity, the manifestation of his own life. And this life-activity he sells to another person in order to secure the necessary means of subsistence. Thus his life-activity is for him only a means to enable him to exist. He works in order to live. He does not reckon labor as part of his life, it is rather a sacrifice of his life.  -- Karl Marx "Wage, Labor, and Capital"