A machine is characterized by sustained, autonomous action. It is set up by human hands and then is more or less set loose from human control. It is designed to come between man and nature, to affect the natural world without requiring or indeed allowing humans to come into contact with it. Such is the clock, which abstracts the measurement of time from the sun and the stars: such is the steam engine, which turns coal into power to move ships or pump water without the intervention of human muscles. A tool, unlike a machine, is not self-sufficient or autonomous in action. It requires the skill of a craftsman and, when handled with skill, permits him to reshape the world in his way.