“What do you mean? It makes total sense!”

“How do I know? It’s obvious! Even a child could understand it.”

One of the most common logical fallacies around is when people wish to persuade you of a fact by logic alone. The argument usually goes like this: in order to convince you of their point of view, they will tell you that their argument makes sense: and that this, on its own, is enough for it to be true. Here are some examples:-

George W Bush and some of his key aides were not in Washington at the time of 9/11. Some people believe that this is evidence of a government conspiracy, as it would have made sense for him to be as far away from the atrocities as possible.

The numbers 5,15,26,34, 35 and 43 are going to come up in the lottery because they have not appeared in any winning tickets for ages. It makes sense to pick these numbers because they are more likely to be coming up soon.

Poison Ivy causes a bad rash upon contact, so based on the homeopathic principle of “like cures like”, it makes sense that it should cure skin rashes.

Reality is smarter than you are

All of these ideas, at least superficially, seem to be based on logic; but logic on its own is not enough. The premises, or assumptions, need to be true too. In many cases, the premises can be hidden or not fully understood – even by the greatest experts on the subject. Throughout history, people have made statements of fact without understanding everything about the phenomenon, with bizarre results. Thus, it was possible to believe that the sun and the planets revolved around the Earth: it made sense because the Earth was assumed to be the centre of the universe. Doctors indulged in bloodletting because it made sense that illness was cause by fluid imbalances.

So, how can we move forward when all we have is logic and we don’t fully understand the premises? Fortunately, there is a hack. We can test it against reality. We can perform an experiment to see what actually happens or we can analyse the available evidence. If we are wrong in our assumptions, reality should be able to tell us.

Michelson and Morley’s famous “failed experiment“, where they they tried unsuccessfully to prove that the speed of light would vary depending on the direction of motion of our planet, was a great example of reality being in dispute with the conventional wisdom of the day. By discovering that light had the same speed in all directions, they were able to show that the common sense of the time (i.e. that light needed a medium – aether – in which to propagate) was demonstrably wrong. Other forces, not then fully understood, were at play. Thousands of experiments like this have given rise to our current understanding of reality. They have often shown our universe to be very different to what common sense would have expected.

The nonsense claim doesn’t work either

Another aspect of the Common Sense Fallacy is the idea that if it does not appear to make sense, it cannot, therefore, be true*. Just as common sense, on its own, cannot be used to establish if something is true, neither can it be used, on its own, to dismiss a proposition out of hand. Creationists will invoke the ultimate 747 gambit to claim that evolution does not make sense (that it’s as unlikely to happen as if a tornado constructed an intact 747 capable of flying around the world) when the evidence (and logic) comprehensively attests to its truth. Some people will claim that global warming doesn’t make sense after experiencing a few cold winters. Quantum mechanics makes no sense at all, even to experts in the field, yet it can be tested with incredible accuracy each and every time.

If in doubt, test it out

Because reality can often confound even the greatest minds, common sense, on its own, is not a good indicator of the truth or falsity of a claim. When someone tries to convince you of the truth of a claim using an appeal as to how “sensible” it seems, be sceptical. To better substantiate a claim you need to be shown reliable evidence. Don’t be fooled. If in doubt, test it out.

* This fallacy is also known as the Argument from Ignorance.