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Common Knowledge

Lost in the Stars


Wandering around on the Internet can lead to some strange discoveries. Here’s a case in point. There is a specific definition of common knowledge used in logic. We generally think we know what common knowledge must be. It’s just the stuff that everybody knows. But that’s not what it actually is.

The stuff that everybody knows is “mutual knowledge”. We all share in that piece of knowledge. What makes it common knowledge is when we know that everybody else knows it. To illustrate this, think about the story of the Emperor’s New Clothes. In that story, the weavers tell the Emperor that they have made him the finest clothes, but they can only be seen by those that aren’t “hopelessly stupid”. So while everyone knows that the Emperor is naked, making it mutual knowledge, but no one knows that everyone knows it, so it isn’t common knowledge.

Of course, one problems with “knowledge” in this sense is that it doesn’t have to be true. Lots of people “know” things that simply aren’t true. Even when it is something that “everybody” knows, if it’s false it has consequences. Logically a false premise can be used to imply anything you like.

But so often we will cling to our false premises with the help of groups that we belong to, that silence the voices of the small children telling us that the Emperor is naked, just so we can continue to hold on to some common knowledge we have grown to love. And all too often an appeal to common knowledge or common sense is more intended to stop you from investigating the truth more deeply.

But simply disagreeing with the common knowledge doesn’t make you a maverick, a Galileo, fighting for truth. All too often the crank fighting against the “system” really is a crank. Yet too often such people are held up as voices for the other “side”.

Science and our legal system have processes for weakening the hold that false knowledge has on us, but to few of us are truly adept at applying those types of techniques in our lives everyday. And what Voltaire said so many years ago is still true, “Common sense is not so common”.

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