If your viewpoint is the history of mathematical proof, then the answer might be "Everything up to the early Greeks." Here's a nice link: "The History and Concept of. Mathematical Proof" by Steven G. Krantz http://www.math.wustl.edu/~sk/eolss.pdf
But if you want to really understand then take a look at the book
Where Mathematics Comes From: How the Embodied Mind Brings Mathematics into Being by G. Lakoff & R. Núñez.
The introduction and first four chapters [PDF] are available at