Roman architecture's three principles: Strength (Firmitas), Function (Utilitas), and Beauty (Venustas).
Vitruvius reduced good building to three demands: firmitas (it must stand), utilitas (it must serve), and venustas (it must delight). Architecture succeeds only when all three hold at once — strength without use is a monument, use without beauty is mere shelter.
The Vitruvian triad unlocks balanced evaluation: a framework that refuses to let any single virtue win, forcing structure, function, and beauty into permanent negotiation. Two thousand years later it still underwrites every argument about form versus function. It is design's oldest reminder that a made thing answers to more than one master at once.