THE MEANING OF REQUIREMENTS

We use the term requirements to denote what are often called functiona l requirements. Requirements are located in the environment, which is distinguished from the machine to be built. A requirement is a conditi on over phenomena of the environment. A specification is a restricted form of requirement, providing enough information for the implementer to build the machine (by programming it) without further environment k nowledge. To describe requirements appropriately we must fit our descr iptions into an appropriate structure. This structure must respect the distinction between the machine and the environment, and the distinct ion between those environment properties that are given (indicative de scriptions) and those that must be achieved by the machine (optative d escriptions). Formalisation is a fundamental problem of requirements e ngineering. Since most environments are parts of the physical world, a nd therefore informal, the formalisation task is inescapable. Some tec hniques are discussed for tackling this task. In particular, the use o f designations is explained, and the distinction between definition an d assertion. By using the smallest possible set of designated terms, a ugmented by appropriate definitions, the developer can create a narrow bridge between the environment and its descriptions in the requiremen ts. In this way a sufficiently faithful approximation. to the informal reality can be obtained.