[Thales](http://en.wikipedia.org/wiki/Thales) (circa 600BC—roughly 50 years before Pythagoras, 200 years before Plato,
and 300 years before Euclid)
certainly knew and reasoned with the concept of a planar *angle*.
Are there earlier historical references to angles in
a mathematical sense?
Or is the origin of the notion lost in the mists of antiquity?