However, as nobles it is reasonable to assume they had staffs of assistants and retainers to help refine and implement their work. The oldest surviving book on architecture, De architectura by the Roman architect Vitruvius describes the design and construction of towns, buildings, clocks, and machines, but provides no information about the organization of the architect's assistants. It is generally accepted that throughout most of human history, most architects were wealthy individuals who derived their primary income from activities other than design and who practiced design as a part-time pursuit, employing assistants on a project-by-project basis.