Scholars have been puzzled by the central argument of MP 1 where the author addresses the basic principle behind the balance and lever. It is not clear what is intended to provide the explanation—the dynamic concepts of force and constraint or the geometrical demonstration. Nor is it clear whether the geometrical part of the argument carries any logical force or has value as a proof. This paper makes a case for the cogency of the argument as a kinematic, not dynamic, account. MP 1 proceeds systematically as it extends the explanatory power of the parallelogram of movements from rectilinear motion to circular motion. Euclid's Elements I.43 provides insight on the author's procedure. His general method is demonstrative, as described in Posterior Analytics I.1.