"Man is the only slave. And he is the only animal who enslaves. He has always been a slave in one form or another, and has always held other slaves in bondage under him in one way or another. In our day, he is always some man's slave for wages, and does that man's work; and this slave has other slaves under him for minor wages, and they do his work. The higher animals are the only ones who exclusively do their own work and provide their own living."

Source: Mark Twain (1973). “What Is Man? and Other Philosophical Writings”, p.84, Univ of California Press