For Leonardo, even buildings - like any other mechanism or structure - are subjected to the rigid, unchanging laws of Nature. He repeatedly underlines the analogy between buildings and the mechanical structure of the body - a comparison that emerges with particular force in his anatomical drawings of the top of the skull and the heart ventricles. The analogy rests on the notion of structures as organisms composed entirely of indispensable, fully integrated parts.
This vision led him to draw a parallel between the physician and the architect: "Just as doctors ... should understand what man is, what life is, what health is ... and in what manner a balance and concordance of elements will preserve health, while their discordance will destroy it... The same is necessary for an invalid building, that is, a doctor-architect who has a good knowledge of what a building is, and from which rules good building derives..."