A disciplined approach to structuring software so that business rules remain independent of frameworks, databases, and delivery mechanisms. Apply these principles when designing system architecture, reviewing module boundaries, or advising on dependency management.
Source code dependencies must point inward -- toward higher-level policies. Nothing in an inner circle can know anything about something in an outer circle. This single rule, applied consistently, produces systems that are testable, independent of frameworks, independent of the UI, independent of the database, and independent of any external agency.
The foundation: Software architecture is about drawing lines -- boundaries -- that separate things that matter from things that are details. Business rules are what matter. Databases, web frameworks, and delivery mechanisms are details. When details depend on policies (not the other way around), you can defer decisions, swap implementations, and test business logic in isolation.