The appeal to common sense – garbage in the guise of gumption

It goes without saying that this tactic is a red flag.

How to recognise this tactic

The perpetrator tries to persuade you to accept or reject a claim based on what’s supposedly “common sense”. Look out for key words such as “Obviously, …”, “Naturally, …”, “Everyone knows …” or “It goes without saying that …”.

There is always a well-known solution to every human problem — neat, plausible, and wrong.

H L Mencken, American writer, 1917

How this tactic is supposed to work

This tactic depends on the notion that common sense is the basis of all correct reasoning. Users may even insist that science is really just common sense at heart.  After all, back in 1854, T H Huxley said:

Science is, I believe, nothing but trained and organised common sense, differing from the latter only as a veteran may differ from the raw recruit.

Their line of argument is that if some complex scientific procedure comes out with conclusions that “defy common sense”, then the science must be wrong.

Why do people use this tactic?

People use this tactic when they don’t accept a scientific conclusion and want you to adopt their position. They either don’t understand the science, don’t want to understand it, or can’t muster good reasons for not accepting it. They promote the common sense explanation as temptingly simple, straightforward and authoritative. They try to distract you from delving deeper into the science by priming you to ignore it.

What’s wrong with this tactic

Basic common sense is often wrong. It tells you that the earth is flat, that heavy things fall faster than light ones, that the sun rotates around the earth, that you should throw water on an oil fire and that stress causes hypertension. Science tells us that all of those conclusions are wrong. Common sense doesn’t predict that you can melt ice by throwing salt onto it, that you can get sunburned on an overcast day or that two brown-eyed parents can have a blue-eyed child. Science can explain all those things. Huxley’s idea that science is highly refined common sense has a lot of support. But the “highly refined” part is very important. Physicist John Ziman puts it this way:

… it is in its conformity to the small print of common sense that science is distinctive. … Taken one by one, the cognitive norms that have to be satisfied – accuracy, specificity, reproducibility, generality, coherence, consistency, rigour, and so on – are all perfectly commonsensical: but they are seldom applied simultaneously outside science.

And philosopher Susan Haack expresses it like this:

… inquiry in the sciences is like empirical inquiry of the most ordinary, everyday kind – only conducted with greater care, detail, precision, and persistence, and often by many people within and across generations … … the evidence with respect to scientific claims and theories is like the evidence with respect to the most ordinary, everyday claims about the world – only denser, more complex, and almost always a pooled resource.

Common sense is often fine in very simple situations where there are few factors involved, and relationships between the factors are linear. (Does the amount of sugar I put in my coffee influence the sweetness?). When things get complex, (Will I get dehydrated from drinking coffee?), science is the way to go. When science and common sense seem to disagree, it’s a pretty safe bet that the science has it right.

What to do when confronted by this tactic

The perpetrator is hoping you’ll automatically defer to the common sense argument. Don’t. Be curious. Demand evidence. Ask if the scientific view differs from the common sense view. If it does, try to find out why scientists are confident in their conclusions despite the apparent contradiction. It’s always possible that the science is wrong, but you can only say that after critically examining the scientific argument. Don’t accept the excuse that scientists are scheming to cover up the truth, and look for red flags such as rooster syndrome, cherry-picking and apophenia.

Variations and related tactics

The appeal to common sense is similar to the appeal to common belief or bandwagon fallacy (for example, that antibiotics will cure a cold). The appeal to common belief is a little less persuasive because common sense implies some logical validity, while it is well know that many common beliefs are wrong.

Examples

• Those who deny the reality of human-caused climate change frequently use the appeal to common sense. One of their arguments is that  if CO₂ is causing global warming, then global surface temperatures should be seen to rise in lock-step step with atmospheric CO₂ concentration. They point out that this does not happen, and claim that global warming is a hoax. Their error, of course, comes from ignoring that there are many factors in addition to CO₂ involved, the relationships are not linear, and the climate system is very complex. More at The Conversation. Other examples of the common-sense fallacy by climate-deniers include: Scientists can’t predict the weather, so how can they predict climate changes?; CO2 makes up only0.039% of the atmosphere – such a small amount could not possibly have a warming effect; If climate scientists were serious, they would stop breathing because they exhale carbon dioxide; Unusually heavy snowfalls disprove the existence of global warming.
• A favourite slogan of anti-fluoride campaigners is “Fluoride is rat poison.” If fluoride is used as rat poison, why would we put it into our drinking water? Sounds like common sense, doesn’t it? The situation is much more complex than the anti-fluoriders like to think, and the science is overwhelmingly in favour of water fluoridation. Any substance, even water, can be toxic if sufficient amounts are ingested. Toxicity depends on dose size. Our bodies can deal with small quantities of most substances, as long as they stay below certain limits, with a different limit for each substance. Fluoride is added to drinking water in such trace amounts (around 1 ppm, or 1 mg/L) that it could not result in poisoning. A 20 kg child would need to drink 100 litres of water in one go to get a toxic dose.
• The philosophy of the Vitamin C megadose industry is based on a similar common sense fallacy: If a little vitamin C is good for you, a lot more must be much better, must it not?. This notion assumes a linear relationship between dose and benefits, and ignores the fact that toxicity is dose-related. The real picture is, of course, more complex. There is little scientific evidence that vitamin C megadoses have any benefit for preventing colds or treating cancer or any other condition (see here, here, here, here, here, here, here, here and here. For most people, megadoses of vitamins of any type are a waste of time and money.
• Update: Here’s an audience member at the Bill Nye/Ken Ham Creation vs Evolution debate proudly using the common-sense fallacy: Of course, the reality is that both humans and present-day apes descended from common ancestors.

Further reading

Crafty contrarians and wily watchdogs – donning the mantle of shrewdness

Scientific knowledge suppresses but does not supplant earlier intuitions by Andrew Shtulman  and Joshua Valcarcel in Cognition.

Why scientific truth may hurt by Adam Rutherford at The Guardian.

We can’t trust common sense but we can trust science by Peter Ellerton at The Conversation.

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H L Mencken’s quote is from “The Divine Afflatus” in New York Evening Mail (16 November 1917). Greg Williams’ Buttered Cat cartoon is from WikiMedia Commons. T H Huxley’s quote is from “On the Educational Value of the Natural History Sciences” (1854). John Ziman’s quote is from Real Science: What it is, and what it means, Prometheus Books, New York, p 314 Susan Haack’s quote is from Defending Science – within reason, Prometheus Books, New York, p iv The Global Temperature and Carbon Dioxide graph is from NCDC. The creationist question photo by is Matt Stopera from BuzzFeed

This is one of ScienceOrNot’s Science red flags. See them all here.