A Quick Lesson in Logic

"But that's the way we've always done it!"

By Scot DuFour  |   Nov 28, 2017

Humanity has made some mistakes along the way. I hope I don’t have to actually create a list of all the things we’ve been wrong about to get my point across on this issue. But in the past we have promoted slavery, prevented women from voting, thought the earth was flat, murdered people for being witches, and too many other wrongs to list.

I routinely hear cops talk about how rational and logical they think they are. But in my experience, we’re as prone to poor reasoning as anyone else. We stand by our practices and convictions despite the overwhelming evidence otherwise. Knowing some of the most common errors in reasoning can help everyone from being fooled by bad arguments.

A Quick Lesson in Logic

Cops love to use the word “logical” and then go on to make completely illogical decisions or arguments. Logic is an actual academic field of study within philosophy. It’s used to make, analyze, and critique arguments of all kinds.

Sherlock Holmes liked to boast about his deductive reasoning skills but even he got it wrong. He was actually using inductive reasoning. Logic goes to great length to explain concepts like validity, relevance, soundness, acceptability, groundedness, and other aspects of arguments. I specifically want to discuss one aspect of logic: informal fallacies.

Fallacies 101

Informal fallacies are mistakes in reasoning so common they have been given names. Philosophers are taught to identify them because they can be hard to spot and are sometimes very persuasive. You likely know many informal fallacies already, some of them include: ad hominem, straw man, appeal to ignorance, slippery slope, and appeal to popularity. Each of these fallacies uses irrelevant factors to make an argument for a specific conclusion. To understand this idea better we need to see how an argument is structured.

One of the first arguments philosophy students are taught in logic class is this:

P1 – All men are mortal.

P2 – Socrates is a man.

C – Socrates is mortal.

This argument is a deductive argument and uses two premises to reach a conclusion. There is no limit on how many premises and conclusions we can use in real-life arguments but this is an easy place to start. Since the argument is deductive it guarantees that the conclusion is true as long as the premises are true.

There are three main categories of ways we can inspect this argument: acceptability, relevance, and groundedness. In the argument above, are the two premises acceptable? Is there good reason to accept them as true? Are the two premises relevant to the conclusion? And finally, do the premises ground the conclusion? Do the premises provide you with enough information to say the conclusion is true?

That’s How We’ve Always Done It”

What’s logically wrong with your boss or coworker insisting that something be done a particular way and justifying it because that’s the way it was done in the past? Besides being annoying, it’s actually a mistake in reasoning. The argument might look like this:

P1 – We have always done X in this department

P2 – Past practices are always best practices

C – We must continue to do X in this department

Are the premises acceptable? You can look at the historical practices of your department and determine if P1 is acceptable. If your department has a history of doing X then it’s probably acceptable. Is P2 acceptable? No. You can point to numerous examples of times where maintaining the status quo was a poor decision. Are the premises relevant to the conclusion? Yes. They are both related to the conclusion. Do the premises ground the conclusion and provide ample evidence? No.

This argument is a version of the Appeal to Popularity fallacy and is often called the Appeal to Tradition. This argument not only uses faulty reasoning but is also just plain lazy.


There are a lot of fallacies and even the best of us fall into the trap of using them. Making good decisions as an individual, as a supervisor, and as an agency require us to honestly critique our arguments for and against our actions and policies. There are many places online that you can find lists of informal fallacies to read and then reflect on times you may have used those types of arguments. After you read up on the informal fallacies you can easily identify the next time your boss uses a common mistake in their reasoning. How you inform them of their mistake is up to you.

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Scot DuFour

Scot DuFour

Scot DuFour began his career in law enforcement with the Phoenix, Arizona Police Department in 2004 and worked primarily on patrol in South Phoenix. In 2008, Scot moved to Colorado and transferred to the Aurora, Colorado Police Department where he was selected for the Vice and Narcotics Section in 2011. Scot is currently a Task Force Officer with the Drug Enforcement Administration’s OCDETF Strike Force. Scot has an AAS in Law Enforcement Technology with honors, a BA in Philosophy with a concentration in ethics, magna cum laude, and a Master’s Degree in Criminal Justice with honors.
Scot DuFour

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