Physical force is as puzzling as Saint Augustine found time to be. We know what force is if no one asks us, but if we want to explain it to someone, we don't know. We know force as a bodily sensation, and we believe it is implicated in efficient causation; we have ways of measuring it, but we cannot explain its essence. That alone would account for the absence of a systematic metaphysics of force in Descartes’ writings, which in turn would account for it being the most intractable problem posed by his physics. He never defines force per se, yet he has a concept of extension and of motion and therefore a conception of physical force expressible as a mathematical quantity. Forces are measurable in terms of the same independent variables, which enables Descartes to compare forces of the same category. For the ontological ground of forces he appeals to God's continuous creative and conserving activity in the physical world (see concurrence versus conservation, divine). It is no surprise that complete agreement among Descartes scholars on the topic of force is in short supply.

Descartes categorizes force in two ways. First, in statics or traditional mechanics, the key notion is the force or effort required to raise a body against gravity, either vertically or along lines inclined to the vertical. The measure of such a force is the product of the body's weight and the vertical distance through which it is raised. It follows that the same force will raise bodies through vertical distances inversely proportional to their weights, a rule that in various forms goes back to antiquity. Descartes’ statement of this rule may be called his “General Statical Principle” (GSP) (Gabbey 1993; Slowik 2002, 113–17).

The second category of force applies in domains outside traditional mechanics: collisions between bodies, stones in slings, rigid-body rotation, planetary motion, ballistic phenomena, and free fall under gravity. By the first law of nature, a body in motion has a force to remain in motion; at rest it has a force to remain at rest (see inertia). The quantity of motion, given by the speed times the body's volume or corporeal quantity, measures the motive force. The body's “rest force,” peculiar to Descartes, is the force with which a body at rest resists any action that would dislodge it from its state of rest.