Leon Battista Alberti was not only the author of the first Renaissance treatises on painting, sculpture, and architecture. He was also the first to succeed in writing a systematic account of the principles of building considered as an art, essential for the well-being and preservation of the state and its citizens, and worthy of the interest of wealthy patrons and erudite humanists. In this paper I argue that Alberti succeeded in doing so not because, as is generally assumed, he set out towrite a treatise about the principles of classical architecture based on Vitruvius, but because he presented an analysis of architecture based on an a-historical and a-stylistic conceptual framework, in which Nature instead of the ancients was the ultimate authority. Although he did not consider architecture to be a language, this framework did enable him to use the methodological and structural models offered by classical treatises on rhetoric to construct his own systematic treatment of architecture considered as an art. (This article was held over from the Spring issue, which contained material from a conference on the theme 'Are there evolving principles in architecture?'). © 1999 The Journal of Architecture.