What are the minimum reading requirements for understanding Newton's Principia Mathematica Naturalis Philosophae? For a long time, the self-contained nature of Newton's Principia has intrigued me. At a glance, it looks as if Euclid's Elements would be the only required reading for understanding his arguments. But it's still pretty tough going. Are there any lesser-known works from his time or before his time (I'm not looking for something to explain him to me, I want to read him and understand his arguments from first principles, the way he wrote them) that might have been obvious points of reference for people at the time he published, that simply haven't survived the way Euclid has?

See Newton Revisited: An excursion in Euclidean geometry
by Greg Markowsky
http://arxiv.org/abs/0910.4807
