Non-Functional Requirements Revisited

Goal-Oriented Requirements Engineering (GORE) is founded on the premise that functional and non-functional requirements (NFRs) are stakeholder goals to be fulfilled by the system-to-be. Moreover, functional requirements are "hard" goals with clear-cut criteria for fulfillment, while traditionally NFRs are usually "soft" goals (aka softgoals) lacking a clear-cut criterion for success. We argue against this distinction and in favor of a different one: traditional NFRs (e.g., security, reliability, performance, usability etc.) are requirements for qual- ities that existentially depend on the subject they qualify. We give examples in support of our argument, and sketch an abstract syntax and semantics for goal models that follow our proposal.