Before we get to the first rule which applies to St. Georges spear, you need the rule of generic views, and it is: Construct only those visual worlds for which the image is stable. So, by default your visually constructing brain will choose the most likely and consistent interpretation of what you are seeing. In this first rule you "could" be seeing a bent line from the only point of view in which it would look straight to you, however if you move your head just a little the illusion of a straight line would be lost. Rule one says that we will read a straight line in a 2D image as a straight line in 3D. Raphael was having some fun with this and using it to open up spaces in his painting. (-> #4)
