In my opinion the principle and most important part is Firmitatis. With out a solid foundation and structure, the building is at risk of collapse, making any beauty or utility it may hold void and posing a genuine hazard to those using it. Next I believe the second most important aspect of a building is Utilitatis. I place this above Venustatis for one key reason— it is the function of a building that separates it from art. While architecture can be beautiful, it is also a humanist discipline and needs to meet the needs of people first and foremost.