Informal vs Formal Fallacy – What is the Difference?

We as individuals can reason about things and reach conclusions. We make mistakes and so we reach conclusions that are wrong. We call these reasoning mistakes fallacies.

Here is an example.

Last Saturday I cut my lawn. It rained later that day. It was not the first time that happened. The previous time I cut my lawn it also rained later that same day. Clearly cutting my lawn causes rain … right?

Rather obviously not.

I’m attempting to keep this all very easy and accessible and not bury you in lots of complex terminology. Think of this as a gentle introduction.

My example above contains a logical fallacy. I’ve made a mistake in my reasoning. The error is one called “Post hoc ergo propter hoc“. That’s a latin phrase that simply translates as “after this, therefore because of this“. The mistake I’m making is that I did something and then something else happened, so I reasoned that what I did caused what happened. While my example is trivial, silly, and obviously wrong, it is actually a rather common reasoning mistake that very smart people can make. (See Correlation does not imply causation)

You can read more about this specific reasoning mistake on the wikipedia page that describes it.

Interestingly enough, it is what is known as an “informal” fallacy.

What the heck is an informal fallacy?

The mistakes we make when thinking about things, fallacies, are generally divided into two types. These are commonly referred to as “formal” and “informal” fallacies.

If you check the Wikipedia page it explains it like this …

formal fallacy can be expressed neatly in a standard system of logic, such as propositional logic,[2] while an informal fallacyoriginates in an error in reasoning other than an improper logical form.[5] Arguments containing informal fallacies may be formally valid, but still fallacious.[6]

If you are thinking “huh!”, then yes, you are not alone.

Informal Fallacy – Appeal To Authority

To explain, I’ll use another well-known fallacy to illustrate what “informal” means.

There exists a group known as the Heartland Institute. If you work in health care, specifically on the topic of smoking causing cancer, or in climate science, they you will be familiar with these folks and perhaps refer to them as cretinous dishonest wankers. That’s not a political slur, it’s an accurate description. This is the think tank that in the 90’s promoted the idea that smoking tobacco was jolly fine stuff and does not cause cancer. It should of course be no surprise to learn that The Tobacco industry was paying them lots of money to fight tobacco legislation.

These days the oil and gas industry funds them to take a stance against climate science, again to push back against legislation.

One of the strategies that they deploy is the use of “Appeal To Authority”. This is where they publish claims such as the following.

SPOILER ALERT: … They don’t, the authors of this text are lying.

Why Scientists Disagree about Global Warming” sounds credible and concerning. Spoiler Alert, this is not the prevailing scientific consensus, but you would not find that out by reading this.

They are leaning upon a supposed authority, scientists, to promote their false claim.

Permit me to distill it down for you. Rather than rely upon an international team of subject matter experts, evidence, data, and conclusions reached after decades of analysis, instead let’s listen to people who have no relevant climate science expertise or experience, and are working for a right-wing political libertarian institute funded by the Oil and Gas industry. That should give us a truly impartial insight into it all … right? (At the time I wrote about this duff publication – see here)

In other words, the scientific “experts”they cite are not actually scientific experts in this field at all and that also don’t have any credible evidence for their claim.

It is basically the “Appeal to Authority” fallacy, except it is not an honest mistake, but instead is very blatant psychological manipulation that they have been paid for.

Now let me cite a subject matter expert, Michael E. Mann

For more than two decades I was in the crosshairs of climate change deniers, fossil fuel industry groups and those advocating for them – conservative politicians and media outlets. This was part of a larger effort to discredit the science of climate change that is arguably the most well-funded, most organised PR campaign in history. Now we finally have reached the point where it is not credible to deny climate change because people can see it playing out in real time in front of their eyes.

Observer Interview with Michael E Mann

Climate change is indeed real and these corporate interests are lying to you. Michael E Mann confirms this.

Wait“, you might say as you leap up and shout “That’s a fallacy, you are appealing to an authority“.

Yes, I am indeed appealing to an authority. In this case the authority is a climatologist, geophysicist, and also is currently the director of the Earth System Science Center at Pennsylvania State University. He has spent decades gathering and publishing peer-reviewed scientific evidence that confirms the reality of human caused climate change. He has also faced decades of well-funded misinformation campaigns that have all, without exception, been robustly debunked. Quoting him as an authority is not a fallacy. This is because he is an authority.

How can appealing to an authority be both a fallacy and also not a fallacy?

Is this Schrodinger’s fallacy perhaps?

Not at all. What “informal” means when describing a fallacy like this is that it might sometimes be OK, but it can also be wrong.

In this case “Appeal to authority” is a fallacy if your “authority” is not actually an authority on the topic at all. However, if your authority is indeed a well-established subject matter expert then it is not a fallacy and so citing them is a wholly valid stance to take.

The “Post hoc ergo propter hoc” fallacy is also informal.

This is because actions can and do result in consequences. It is of course not always true, we might spot a correlation and assume a causal relationship when there is no causal relationship at all. Another silly example: Every single human who consumed tomatoes between the years 1753 and 1824 is now dead. Clearly tomatoes are deadly to humans.

Informal vs formal

OK, let’s try to sum this up.

An informal fallacy is some reasoning that might or might not be valid. Sometimes it is wholly valid and sometimes it is not.

  • You can cite an authority if they really are an authority. This is not a fallacy. If however your “authority” is not really a subject matter expert of the topic you are discussing, then that is a fallacy.

In contrast to the above, a formal fallacy is a fallacy that is always wrong.

  • You are Human. You are a mammal. I have a pet and that pet is a mammal, therefore my pet is a human. Er no, you can’t reason like that, because if you do then you will always be making a mistake. This is a formal fallacy called Affirming the consequent

Why does any of this matter?

Understanding fallacies is not about having a club to beat people over the head with when debating, but rather is a way to grasp how we can fool ourselves with faulty reasoning. It is a tool we can use as we strive to believe as many true things as possible and also discarding things that are simply not true at all.

You will make mistakes and you will also be deceived and manipulated. Make peace with that reality. You can use an understanding of reasoning mistakes to enable you to potentially recognise how and when you have been fooled and then utilise that insight to make the leap to better more robust conclusions.

I’ll stop there for now. I do hope that you found this, not simply interesting, but that it motivates you to start thinking a bit more about how you think about things.

Further Reading

2 thoughts on “Informal vs Formal Fallacy – What is the Difference?”

  1. I think you’re article is incorrect. “Appeal to Authority” is especially relevant to the example which you stated was valid. It shouldn’t matter how much of an expert someone is on the subject matter. Quoting him instead of proving your argument is lazy and a fallacy.

    Using the scientists reasoning to prove your point is not a fallacy.

    But quoting the scientist’s agreement, rather than his reasoning, is a fallacy no matter how much of an expert the scientist is.

    • We can’t all be experts on every single topic.

      The point was to illustrate the difference between a formal and an informal fallacy. Leaning upon subject matter experts is fine, as long as they really are subject matter experts. If however, we lean upon an individual who may indeed be smart and an expert in something, but what we lean upon then for is not their area of expertise, then that’s a fallacy.


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