Properly Trained Common Sense

Toolbelt of Knowledge: Practices
Skepticism
Listening
Deconstruction
Rationality
Mindfulness
Steel Manning
Common Sense

On this blog, we have not treated common sense too kindly. “Common sense” is what we use to mean “you just know,” without having to go through all of the tedium of proving something. In general, it is a catch-all term for mental shortcuts that get us to answers quickly without getting bogged down in confusion. Because of this, common sense can be incredibly idiotic. However, there are also times when it is wise and practical. Today, we’re going to look at how to use common sense well, so that you come to conclusions that have high probabilities of being correct and useful.

The first type of common sense is a reality check. After you have gone through a process of deduction and calculation, does the answer you get make sense? For instance, suppose you are doing a homework problem to calculate the speed of sound, and the answer you get is 30 miles per hour. Do you turn it in? You may not know what the real answer is, but you do know you don’t hear a sonic boom every time a car speeds up nearby! Common sense says you better check your calculations.

Not considering the consequences of the choices we make or put off making can be considered a breach of common sense. If you’re making rice and you fill the pot with grains, it will overflow as it absorbs the water. If you have a pain in your wisdom tooth, waiting to see if it goes away on its own is not worth the risk that it’s infected. When you vote for politicians and leaders, you might want to avoid the ones who like to beat people down. Actions and consequences. Common sense.

Our next use of common sense is to accept true statements that are extremely difficult or impossible to prove. There is a form of argument called the syllogism, which is two premises and a conclusion. An example would be, “All men are human, Frederick Douglass was a man, therefore Frederick Douglass was human.” In the abstract, the syllogism looks like this:

A: If B, then C.
B: B.
C: Therefore, C.

It is obvious that if someone accepts both A and B, they must conclude C to be true as well.

But think about the statement in bold. It is a premise in itself, a hidden premise of the argument. So let’s bring it out of its hiding place, and add it to the syllogism as a premise Z.

Z: If A and B, then C.
A: If B, then C.
B: B.
C. Therefore, C.

There we go. Now we know the whole truth: if someone accepts A, B, and the hidden premise Z, they must conclude C to be true as well.

Oh no! In bringing out the hidden premise to prove the syllogism, we have discovered yet another premise! X: If Z, A, and B, then C. It becomes clear that there is a pattern: for every hidden premise we find, there is yet another premise hidden behind it. Rather than two premises and a conclusion, the syllogism in its true form has an infinite number of premises!

This is an interesting puzzle for the philosophy of logic. But for our everyday problem solving, the two-premise syllogism is good enough, and it is fine to act as if it is absolutely proven to be true. This is our next use of common sense: to take the improvable foundations of logic as if they are proven to be true.

Even when an argument is logically sound, common sense can sometimes veto one or more of its premises. For instance, we can go to the classic example of a faulty syllogism, “All men have beards. Socrates was a man. Therefore, Socrates had a beard.” Our common sense says wait a minute, only some men have beards, not all of them! Despite the argument being formally valid, the first premise is false, meaning the conclusion is invalid, and we can’t know if Socrates had a beard without more information.

However, we must remember, just because something is common sense doesn’t necessarily mean it’s true. At first glance, quantum physics seems to go against common sense, with photons and electrons behaving sometimes as particles and sometimes as waves, and seeming to teleport from place to place. But there is an enormous amount of theoretical and experimental evidence pointing to quantum physics being true, and we have a large amount of technology, such as the laser, that would not function otherwise. Therefore, despite quantum physics going against common sense, we have every reason to believe it is true.

This illustrates the problem with common sense: sometimes it is wrong. If we hear an idea that goes against our common sense, it is important to hear the arguments supporting it, and to give those arguments a good mull over, with the attitude that we might allow ourselves to be convinced to let go of our common sense belief.

Perhaps the worst danger of invoking common sense is to avoid looking at a question in its full complexity, and make out anyone who disagrees with us as fools. We all know of people who have defended their religious or political beliefs by saying, “it’s common sense,” brushing us off, and sending the message that because we don’t agree with their “common sense” view, our thoughts on the matter aren’t worth hearing. That’s not common sense, it’s stubbornness, and we must keep ourselves accountable not to fall into that kind of behavior.

Like rationality, common sense is not something we automatically have. In order not to cause more problems than it solves, common sense must be trained, and a great way to do that is to practice all of the other skills in the Toolbelt of Knowledge.