De Motu Corporum in Gyrum - Commentaries On The Contents

Commentaries On The Contents

At some points in 'De Motu', Newton depends on matters proved being used in practice as a basis for regarding their converses as also proved. This has been seen as especially so in regard to 'Problem 3'. Newton's style of demonstration in all his writings was rather brief in places; he appeared to assume that certain steps would be found self-evident or obvious. In 'De Motu', as in the first edition of the Principia, Newton did not specifically state a basis for extending the proofs to the converse. The proof of the converse here depends on its being apparent that there is a uniqueness relation, i.e. that in any given setup, only one orbit corresponds to one given and specified set of force/velocity/starting position. Newton added a mention of this kind into the second edition of the Principia, as a Corollary to Propositions 11-13, in response to criticism of this sort made during his lifetime.

A significant scholarly controversy has existed over the question whether and how far these extensions to the converse, and the associated uniqueness statements, are self-evident and obvious or not. (There is no suggestion that the converses are not true, or that they were not stated by Newton, the argument has been over whether Newton's proofs were satisfactory or not.)