Proposition 7 of book one appears incomplete in its current form, so that strictly speaking Archimedes in the first book demonstrates the law of the lever for commensurable magnitudes only.[1][2] The second book of On the Equilibrium of Planes does not suffer from these issues because, with the exception of the first proposition, the lever is not treated at all.[3] Perhaps more concerning is that there is no definition of centre of gravity anywhere in Archimedes' extant works, make it difficult to see the logical structure of some of his arguments in On the Equilibrium of Planes.[4][5]