My definitions for software architecture are an elaborated version of those within the Perry and Wolf [105] model, except that I exclude rationale. Although rationale is an important aspect of software architecture research and of architectural description in particular, including it within the definition of software architecture would imply that design documentation is part of the run-time system. The presence or absence of rationale can influence the evolution of an architecture, but, once constituted, the architecture is independent of its reasons for being. Reflective systems [80] can use the characteristics of past performance to change future behavior, but in doing so they are replacing one lower-level architecture with another lower-level architecture, rather than encompassing rationale within those architectures.