The "paradox of the active user" is a concept introduced by John M. Carroll and Mary Beth Rosson (then at IBM, now at Penn State) to explain a common observation in several user studies done at the IBM User Interface Institute in the early 1980s (later confirmed by many other studies, including my own): Users never read manuals but start using the software immediately. They are motivated to get started and to get their immediate task done: they don't care about the system as such and don't want to spend time up front on getting established, set up, or going through learning packages.
The "paradox of the active user" is a paradox because users would save time in the long term by taking some initial time to optimize the system and learn more about it. But that's not how people behave in the real world, so we cannot allow engineers to build products for an idealized rational user when real humans are irrational: we must design for the way users actually behave.
Carroll, J.M. and Rosson, M.B. (1987). The paradox of the active user. In J.M. Carroll (Ed.), Interfacing Thought: Cognitive Aspects of Human-Computer Interaction (Cambridge, MA: MIT Press).
11 years later Dr. Carroll edited a book with case studies of how designers can employ the fundamental insights about minimalism: Minimalism Beyond the Nurnberg Funnel (Cambridge, MA: MIT Press).