What did Gottfried Wilhelm Leibniz mean by force in his account of motion?

In his account of motion, Gottfried Wilhelm Leibniz moved away from the Cartesian view that motion could be defined solely by local displacement. Instead, he argued that visible movement must be the result of an underlying force.1

Leibniz’s critique of the Cartesian formulation of the laws of motion led him to found a new field known as dynamics. Central to this shift was his substitution of kinetic energy for the conservation of movement. By positing that concepts of extension and motion contained an imaginary element, Leibniz concluded that the basic laws of nature could not be derived merely from studying the nature of extension itself. Consequently, he viewed force as the essential, active principle required to explain and predict the course of physical phenomena.1