Cromwell’s Rule

CromwellI beseech you, in the bowels of Christ, think it possible that you may be mistaken.

~Oliver Cromwell, in a letter to the Synod of Scotland

I remember the day I first learned to ask a crucial question during a conversational argument. Locked in a fervent discussion about religion, my interlocutor and I had logged hours of circling around the same issues. I despaired of making any headway. Then it dawned on me—the question! It was this: “Do you think it is possible that you’re wrong?” He answered “no,” and I politely ended the conversation.

Of course, I’ve put the same question to myself: “Is it possible that I’m wrong on this issue?” And I have to say, “yes.” Doesn’t this seem reasonable? After all, the evidence of my fallibility grows daily. I think this assumption, that one could be wrong, is crucial to any productive argument. The only exception being an argument about the fact of my own current existence—if I’m wrong about that one, then all (my) bets are off. Literally.

Cromwell’s Rule

Dennis Lindley (1923-2013), a British statistician, coined the term “Cromwell’s Rule” regarding this crucial assumption. Lindley was concerned about formally calculating probabilities rather than persuasive argument, but his “rule” helps make a good point. You should never assume (unless something is true by definition, like “2+2=4” or “all unmarried men are bachelors”) that something is impossible (or necessarily true), because it renders you practically immune to new evidence to the contrary. Rather, you should leave a little epistemic space for even extremely unlikely scenarios

Doctor WhoHere’s an illustration. In an episode of the popular sci-fi TV show Doctor Who (“Midnight”), a group of tourists travel via ground shuttle to visit the famed emerald waterfalls of the planet Midnight. When the shuttle stalls and knocking sounds are heard on the hull, a scientist on board (Hobbes) tries to reassure them that no living thing could be knocking on the hull because no living thing could survive the lethal “galvanic radiation” that bathes the planet’s surface. As the evidence mounts that the shuttle is under attack by an intelligent being, the scientist simply cannot bring himself to even admit this possibility, despite its being obvious to everyone else aboard. Hobbes remains trapped in his fundamental assumption about life on Midnight, unable to assimilate even the evidence of his own eyes and ears.

Sandy Heads

Religious skeptics, believers and political pundits often fall prey to this same mistake. We could call it intellectual pride, hubris, or plain old stubbornness. Utterly convinced of our position, we blockade ourselves against new evidence like the proverbial ostrich with its head in the stand. Even if it turns out that our belief is true in this instance, the habit of ignoring Cromwell’s Rule makes us vulnerable to error in the future. Good thinking and arguing, for those who are genuine truth-seekers, must include some openness to our own fallibility.