Carlyle

Cromwell's Rule refers to a principle that you should not give probabilities of 1 to any event that is not demonstrable by logic to be true, and never to give probability 0 to any event unless it can be logically shown to be false - see page 91 of Understanding Uncertainty by Dennis Lindley. It comes from Cromwell's appeal to the Church of Scotland to 'think it possible you may be mistaken'.