Vitruvius is famous for asserting in his book De architectura that a structure must exhibit the three qualities of firmitas, utilitas, venustas – that is, it must be solid, useful, beautiful. These are sometimes termed the Vitruvian virtues or the Vitruvian Triad. According to Vitruvius, architecture is an imitation of nature. As birds and bees built their nests, so humans constructed housing from natural materials, that gave them shelter against the elements. When perfecting this art of building, the Greeks invented the architectural orders: Doric, Ionic and Corinthian. It gave them a sense of proportion, culminating in understanding the proportions of the greatest work of art: the human body. This led Vitruvius in defining his Vitruvian Man, as drawn later by Leonardo da Vinci: the human body inscribed in the circle and the square (the fundamental geometric patterns of the cosmic order).