A common and effective computer security practice is
to keep sensitive information on computers that are locked up and disconnected
from the world.
I don’t propose that but this model can serve as metaphor in our design space.
Thus the problem is solved in both theory and practice.
The isolated computer was the original ideal towards which timesharing was directed.
That ideal was largely met. The virtual
machine provides an even more literal illusion of an isolated machine.
The mathematical theory of decision procedures prescribes running two computations simultaneously on the same Turing machine when it is known that at least one will produce the result that you need, but it is not known which.
To do this one writes a Turing machine program that simulates two Turing machines simultaneously.
The metaphor may be relevant to graphic presentation of security arrangements.
The cut and paste metaphor must be coordinated with the separated machine