What makes the computer's representation special is that it can be manipulated so rapidly without direct human intervention. Once the program is determined and the machine set to work, the electrons fly until an answer is produced. An abacus can produce an answer mechanically by means of a person who unthinkingly slides the counters according to the rules. And yet the very fact that a human being is needed to push the counters suggests a close link between man and machine. The abacus is a tool rather than a machine, for it extends human technical capabilities while remaining intimately under human control. A machine runs more or less under its own control, with its own sense of purpose and its own inanimate source of power.