One Fundamental Mystery
The Canon asserts that it is unquestionable that there was a canonical art of building, and that it depended on one fundamental mystery. As direct evidence of the old mystical rule of building, the author presents two illustrations taken from Cesarino's edition of Vitruvius (a first century, Roman architect), published in 1521, which depicts the elevation and floor plan for the Milan Cathedral, claiming that they are "an most important illustration of the principles of architecture of all peoples".
According to the book, the geometric figure forming the plan of the Milan Cathedral is the rhombus, invisibly introduced into the plan as the mystical complement of the cross, emblems of the double soul of the Universe, of the two mysterious factors of existence.