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?
Superficially, it looks like Newton knew his works on geometry very thoroughly, or could guess most of what was in it. Newton does mention Pappus a bit. And besides all that, the diagrams in Pappus and the diagrams in the Principia just look similar.
Which is what made me look for the connection. I happened to be browsing through Book IV of Pappus's works, the section dealing with plane geometry and thought, "wow, that sure looks a lot like Newton's stuff."
But no, Newton avidly read Pappus, according to a paper linking Newton to Pappus:
(long quote by Pappus follows; pdf here)
See Newton Revisited: An excursion in Euclidean geometry
by Greg Markowsky