Marr's Levels of Analysis
Marr's three levels of analysis [cite key="marr1982"] promotes the idea that complex systems such as the brain, a computer or human behaviour should be understood at different levels. Marr's framework proved to be an elegant and popular way of reasoning about complex systems, and in the context of machine learning and statistics, remains an intuitive framework that is often used when describing probabilistic models of cognition and perceptual systems.