 Leibniz (among others), as we saw, made a corresponding distinction: moving force, the power of a body to change the motion of another, was determined by velocity. Leibniz therefore distinguished this force as an active power, fundamentally different from the passive power of a resting body to resist any change of position. Newton, in contrast, as he developed the Principia, and recognized the existence of a class of indistinguishable relative spaces, gradually came to understand the “force of inertia” as what we would call a Galilei-invariant quantity. Impetus and resistance were therefore recognized as appearances of that invariant quantity in different frames of reference: