 […] since one cannot proceed in reasoning about natural things that are brought about by motion from the effects to the causes without a knowledge of those things that follow from that kind of motion; and since one cannot proceed to the consequences of motions without a knowledge of quantity, which is geometry; nothing can be demonstrated by physics without something also being demonstrated a priori. Therefore physics (I mean true physics) [vera physica], that depends on geometry, is usually numbered among the mixed mathematics [mathematicas mixtas]. [.] Therefore those mathematics are pure which (like geometry and arithmetic) revolve around quantities in the abstract so that work [in them] requires no knowledge of the subject; those mathematics are mixed, in truth, which in their reasoning some quality of the subject is also considered, as is the case with astronomy, music, physics, and the parts of physics that can vary on account of the variety of species and the parts of the universe. (MC 42; OL II.93)