Before 1700 there were two main kinds of energy used by human beings: heat and work. (Light came mainly from heat.) People burned wood or coal to keep warm and cook food; and they used their muscles, or those of horses and oxen, or rarely a water wheel or a windmill, to move things, to do work. These two kinds of energy were separate: wood and coal did no mechanical work; wind, water and oxen did no warming.
