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The Freedom software requirements process is based on a black box model of a software system, where a black box is characterized as only that information about the system that is knowable (i.e., "visible") external to the system. Dr. Harlan Mills advocated modeling the black box information of a software system using a stimulus-response paradigm, where
with state being equivalent to the prior stimulus history of the system.
Additionally, Freedom recognizes that all details of the software external interface are externally visible information, and are also part of the black box description of a software system. The software external interface comprises the external artifacts of the system by which stimuli are detected and responses are projected back.
The black box view of a software system therefore consists of
Although Freedom is a software development methodology, its black box based requirements process is very general. Hence, with minor modification, the Freedom requirements process is applicable to specification of requirements for many types of hardware systems as well.