 Newton did try to clarify his method of characterizing forces. If one brackets the question of how to understand forces as ephemeral actions that do not persist after causal interactions have ceased, one can make progress by conceiving of forces as quantities. In particular, since Newton’s eight definitions and three laws indicate that forces are proportional to mass and to acceleration, and since mass—or the quantity of matter, a concept Newton transformed from its Cartesian origins, where it was understood as a measure of a body’s volume—and acceleration are both quantities that can be measured, Newton gives us a means of measuring forces. This is crucial to his method. If one thinks of forces as measurable quantities, moreover, then one can attempt to identify two seemingly disparate forces as in fact the same force through thinking about measuring them. For instance, in Book III of the Principia, Newton famously argues in proposition five and its scholium that the centripetal force maintaining the planetary orbits is in fact the same as the force of gravity, viz., the force that causes the free fall of objects on earth. This was a revolutionary idea at the time, one rendered possible in the first place by Newton’s way of thinking about forces as quantities. This idea then led Newton to the even more revolutionary view in proposition seven of Book III that all bodies gravitate toward one another in proportion to their quantity of matter. That is, it led him to the idea of universal gravity, a view that shocked many of his Continental readers in its boldness. This helped to unify what were once called superlunary and sublunary phenomena, a unification that was obviously crucial for later research in physics. The idea was enabled by Newton’s abstract way of understanding forces—without conceiving of a force as involving any specific mechanism or type of physical interaction, Newton thought of forces as quantities that are proportional to other features of nature.