Saturday, May 19, 2012

Common Sense

I have seen common sense arguments come up a number of times recently and I have a few things to say about it. By a common sense argument I mean something of the form "X must be true, it is common sense", or perhaps something like "it is obvious that X is true" or as Jimmy would say "I mean, it's like, come on"

This is not an argument, it is a starting point. The appropriate thing to do is to formalize the idea and test it. In this way you can see if your "common sense" was correct, or if you were in fact incorrect. This might seem like a waste of time, especially if you are correct, but would never know for sure if people didn't test these things.

How many times has our common sense been wrong?

  • It seems obvious that heavy objects are accelerated by gravity faster than light objects. 
  • Earth of obviously flat
  • Time is clearly constant no matter how close to a massive object you are
  • Length is constant no matter how fast you are going
All of these things seem reasonable, or they did at one time, and yet they are all false. This is just off the top of my head, anyone have other examples to add to the list?

Whenever I see someone argue that their position is obviously true, I assume that they either haven't looked into it, or they have and the argument is weak. If there was a good argument you would just explain it, if your argument is poor or you don't have a good reason you might just assert your conclusion without sufficient support.

In math we used to jokingly call this proof by intimidation. "This is obvious" has the subtext "If you don't see this you are an idiot." As a teacher grading proofs, I view the word "obvious" as "I don't know how to do this and I hope you won't look too close". Reading apologetics, I pretty much see the same exact thing.


  1. I really like your analogies to things that appear to be "common sense" that are really false.

    The last part about grading homework, I am picturing Euclid not bothering with his proofs and just writing "it's obvious man!" I'm sure he wouldn't have made it far. You might be right when making claims, but you have to have proof and evidence to support it.

    1. The funny thing is that proving obvious things is often very difficult because it can be hard to know what you need to write down. We make people do it to train them to figure out exactly what they are assuming and exactly what they are trying to show. Once you lay those things out finding a path between them is often not too difficult.


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