The idea of active power played central role in the 17th Century philosophy and science. The idea is as follows: if not prevented, bodies necessarily do certain things in virtue of their power. This kind of thought naturally arose from what might properly be called the law of persistence, according to which moving bodies continue their motion unchanged if no new external force intervenes.1 What bodies do in virtue of their power was called actions, and in terms of actions such things as resistance, pressure and affections were explained. What is this active power? One of the main aims of philosophers in the 17th and 18th Centuries was to find a good answer to this question.