Software Verification and System Assurance

Littlewood introduced the idea that software may be possibly perfect and that we can contemplate its probability of (im)perfection. We review this idea and show how it provides a bridge between correctness, which is the goal of software verification (and especially formal verification), and the probabilistic properties such as reliability that are the targets for system-level assurance. We enumerate the hazards to formal verification, consider how each of these may be countered, and propose relative weightings that an assessor may employ in assigning a probability of perfection.