From Wikipedia
A History Of European Art
Lecture Twelve
The person generally credited with bringing about the Renaissance view of architecture is Filippo Brunelleschi, (1377–1446). The underlying feature of the work of Brunelleschi was "order".
In the early 1400s Brunelleschi began to look at the world to see what the rules were that governed one's way of seeing. He observed that the way one sees regular structures such as the Baptistery of Florence and the tiled pavement surrounding it follows a mathematical order—linear perspective.
The buildings remaining among the ruins of ancient Rome appeared to respect a simple mathematical order in the way that Gothic buildings did not. One incontrovertible rule governed all Ancient Roman architecture—a semi-circular arch is exactly twice as wide as it is high. A fixed proportion with implications of such magnitude occurred nowhere in Gothic architecture. A Gothic pointed arch could be extended upwards or flattened to any proportion that suited the location. Arches of differing angles frequently occurred within the same structure. No set rules of proportion applied.

The church of San Lorenzo