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What is common sense?

June 30, 2012

Politicians often say that something is just common sense or common knowledge, but what is that?  In its most reduced form, it’s our basic understanding about how our world works.  The system of geometry, for example, begins with a series of basic assumptions.  These are statements like `you can draw a straight line through any two points’, or `parallel lines never meet’.  They are the foundation on which the rest of the system is built.

More generally, common sense is something that everybody knows, something that requires no proof.  It’s a self-evident truth, as in `we hold these truths to be self-evident’.  Are they really as fundamental as we might think?

Common sense can change.  Five hundred years ago, everybody knew that the circulation of water in the earth must be the same as the circulation of blood in the human body.   Leonardo da Vinci based some of his scientific research on this concept, one that was an accepted belief at the time.  Now we know that this analogy was incorrect.  This item of common knowledge has disappeared.

I recently noted that a Canadian politician cited the old story of the beaver biting off his own testicles to escape pursuit.  This statement did provoke a great deal of laughter in the house.  However, the story was proven wrong 400 years ago.  Nobody believes it anymore.  It’s another example of common sense that has changed.

In reality, common sense changes all the time.  It changes whenever our understanding of how the world works also changes.  Common sense depends on the environment in which you are living.  This means that different groups of people may have different concepts of common sense.  You should be suspicious if a politician invokes common sense.  This usually means “I’m not going to offer any proof”.  Analogies are the most dangerous of this dubious form.  Why should household economics be equivalent to the economics of a country?  Why should corporations should be treated the same as individuals?  It’s not just common sense.

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