Virtually Real Computers are Quantum Computers Imagined 42BTrue?
What an excellent article, Kevlin. A Real Gem.
"A unit test is a test of behaviour (normally functional) whose success or failure is wholly determined by the correctness of the test and the correctness of the unit under test."
I would agree with that and it is further enhanced and endorsed/enriched with a Third Party, Positive Reinforcement.... however, it does not necessarily require such Enrichment/HyperRadioProActivity to continue its Progression if the correctness of the test and the correctness of the unit under test, is not hindered from further Testing/Progression.
It may be that the unit test/test of behaviour is Unique and therefore Third Party Action may be hindered by a requirement for them to consider their Reaction/ProAction in that it may also have to be a Unique and Innovative Element which would in Effect, Create a new Component.
Of Course, in some Intelligently Designed Programs, such Creation is exactly what the Programmer is Testing for in what is in Essence a Continual BetaTest of Systems...... to Prove a FailSafe Defaulted System which is New and Unique.
In such a case, .."In other words, the unit in question can be isolated at a level below the whole system and its interactions with the environment." .... will not be applicable, as it will actually sit above the whole system, immune to interactions by virtue of the fact that the system below, which would be the status quo system being unit tested/tested by the unit test, has chosen not to show any action at all, either in reaction [negative and obstructive] or proaction [positive and supportive] It will therefore be free to BetaTest further steps in the Program/Programming, which would prove the Enrichment Algorithm Driving the Program is FailSafe Defaulted in Third Party Positive Reinforcement Theory for No Reaction is equally Valid as a Driver in the absence of a Positive and Supportive ProAction should an Element or Component or even a whole new Operating System be Created for BetaTesting.
Such is Magical Mystery Turing in Advanced Artificial Intelligence Concepts for Global Operating Devices.