Logical Fallacies


Post hoc fallacyLast night at council I referred to seeing what I believed was a >post hoc fallacy in a report, or more properly a post hoc ergo propter hoc fallacy. Yeah, I probably annoyed some folks in the audience because I used Latin words and that confused them. But hey, they already think I’m a jerk because I can spell words like egregious and nefarious without using spellcheck, so I doubt I lost any votes over it.

It means an error in assigning a causal relationship between two or more coincidental events. For example:

  1. I combed my hair a different way and I came down with a cold.
  2. After a few days, I combed my hair the old way and the cold went away.
  3. Therefore combing my hair a different way gave me a cold.

This is an easy one to see through, but you’d be surprised how many people apply this logical fallacy to their thinking. For example, the typical chemtrail conspiracy theory:

  1. I saw contrails from a plane overhead.
  2. My skin got itchy afterwards.
  3. Therefore the government is spraying something from jet planes that is making me sick.

Which pretty much sums up the whole nonsense around chemtrails. People naturally look for events that explain what they already believe to be true (confirmation bias). When causality does not exist between events, what you often find is merely wishful thinking.

Post hoc fallacies are part of a group of related informal, logical fallacies that fall under the general non causa pro causa group.* The Fallacy Files describes this category as:

…the most general fallacy of reasoning to conclusions about causality. Some authors describe it as inferring that something is the cause of something else when it isn’t, an interpretation encouraged by the fallacy’s names. However, inferring a false causal relation is often just a mistake, and it can be the result of reasoning which is as cogent as can be, since all reasoning to causal conclusions is ultimately inductive.

The subcategories of non causa fallacies include:

  • Cum Hoc, Ergo Propter Hoc
  • Post Hoc, Ergo Propter Hoc
  • The Regression Fallacy
  • Texas Sharpshooter Fallacy

Cum hoc fallacies are similar to post hoc, except that where post hoc events happen after, cum hoc events happen with or at the same time. As in: combing my hair gives me a cold because I sneeze more when I comb.

Regression fallacies mean to return to an earlier state (or the mean) to identify a cause, often called a special kind of post hoc fallacy. As in, leaving my hair the old way is luckier because I won $10 on the lottery when it was combed like that.

The Texas Sharpshooter fallacy is one we’ve all seen in politics, particularly during election campaigns (locally it’s been frequently used this election). The myth about “block voting” by a select group on council is a typical “sharpshooter” fallacy. The Fallacy Files describes it:

The Texas sharpshooter is a fabled marksman who fires his gun randomly at the side of a barn, then paints a bullseye around the spot where the most bullet holes cluster.

Wikipedia describes it as:

…committed when differences in data are ignored, but similarities are stressed. From this reasoning a false conclusion is inferred… The Texas sharpshooter fallacy often arises when a person has a large amount of data at their disposal, but only focuses on a small subset of that data. Some factor other than the one attributed may give all the elements in that subset some kind of common property (or pair of common properties, when arguing for correlation). If the person attempts to account for the likelihood of finding some subset in the large data with some common property by a factor other than its actual cause, then that person is likely committing a Texas Sharpshooter fallacy.

You’ve probably seen this on some of the angry bloggers’ sites and in some attack ads: using selection quotes from emails, select pieces of financial data, voting clusters chosen from an entire term, rather than giving the whole picture or examining underlying events.

You can always find some correlation that fits your preconceptions this way, although this also brings into play the confirmation bias, another fallacy commonly seen during this election campaign:

…the tendency to search for, interpret, or prioritize information in a way that confirms one’s beliefs or hypotheses

Then there’s the all-too-common straw man fallacy the bloggers use to discredit anyone they don’t like:


The straw man fallacy means, as Wikipedia tells us:

…a common type of argument and is an informal fallacy based on the misrepresentation of an opponent’s argument.[1] To be successful, a straw man argument requires that the audience be ignorant or uninformed of the original argument.

And as Nizkor describes it:

The Straw Man fallacy is committed when a person simply ignores a person’s actual position and substitutes a distorted, exaggerated or misrepresented version of that position.

And on Your Logical Fallacy as:

Misrepresenting someone’s argument to make it easier to attack.

The most commonly used tactic by the bloggers, however is the ad hominem argument, or personal attack:

Ad hominem

…a form of criticism directed at something about the person one is criticizing, rather than something (potentially, at least) independent of that person. …a fallacy in which a claim or argument is dismissed on the basis of some irrelevant fact or supposition about the author or the person being criticized.

What the ad hominem argument lacks in logic it makes up for in crudity. Ad hominem is what Councillor Hull resorted to last night: unable to debate my concerns about the report, he simply flung insults my way (it might equally be argued he was using straw-man tactics instead, but that’s a semantic debate I’ll leave for now…).

Ad hominem is frequently paired with the tu quoque argument by the bloggers: answering criticism with criticism (with a side dish of ad hominem insult tossed in):

…an informal logical fallacy that intends to discredit the opponent’s position by asserting the opponent’s failure to act consistently in accordance with that position. It attempts to show that a criticism or objection applies equally to the person making it. This attempts to dismiss opponent’s position based on criticism of the opponent’s inconsistency and not the position presented.

A related fallacy is the “genetic fallacy“:

…a conclusion is suggested based solely on something or someone’s origin rather than its current meaning or context. This overlooks any difference to be found in the present situation, typically transferring the positive or negative esteem from the earlier context. The fallacy therefore fails to assess the claim on its merit.

In other words, as the bloggers usually put it: we don’t like councillor Chadwick. Therefore anything councillor Chadwick says must be untrue. Substitute the incumbent-of-choice’s name here and the fallacy is still the same.

Another fallacy is the argument from consensus, also known as the “bandwagon fallacy” and “authority of the many.” It assumes that popularity makes something correct or valid (and conversely that unpopularity makes it invalid or incorrect). Heard at one door while campaigning: “My gang doesn’t like what this council has been doing.” That’s a bandwagon fallacy: we don’t like it so it must be wrong.

Facebook works on this premise: if you get enough people to “like” you on Facebook, you are led to believe that is a sign of personal popularity. It’s also common on the blogs in the “me and my friends believe…” approach – as seen in the fallacious debt figures being thrown around, as if the group belief in the fictitious numbers has greater validity than actual facts.

Councillor Hull’s comments about a prior process belong to the “denying the antecedent” fallacy, the argument for which goes something like this:

  1. If we build the multi-use facility I want, then the process we followed was correct.
  2. We didn’t build the multi-use facility I wanted.
  3. Therefore, the process we followed was incorrect.

As Wikipedia defines this fallacy, “…arguments of this form do not give good reason to establish their conclusions, even if their premises are true.” As Logically Fallacious puts it:

The arguer has committed a formal fallacy, and the argument is invalid because the truth of the premises does not guarantee the truth of the conclusion.

In point of fact, council followed the process recommended by staff when deciding to build our new rec facilities. Not agreeing with the subsequent decision does not mean the process was flawed. To suggest it was is to attack staff’s competence and ability, not council’s choice. It’s just another logical fallacy.

These fallacies in reasoning and logic usually lead to an “ignoratio elenchi” or irrelevant conclusion, “…presenting an argument that may or may not be logically valid, but fails nonetheless to address the issue in question.” Hence my introduction of the Latin phrase mentioned at the beginning of this post.

Logical fallacies


* Some of these (and others) are listed in the article on the “Top 10 Logical Fallacies in Politics.” You can read about these and other logical fallacies in politics in Thomas Cathcart’s and Daniel Klein’s entertaining book, Aristotle and an Aardvark Go to Washington.

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  1. abbottaerospace

    Hmmm – that certainly looks like a ad-hominem attack again Councillor Hull…..

    Sadly there is not a tea party in Canada and people who may agree with them certainly hold no power here – you can see this from the excessive municipal, provincial and federal debt.

    As to the wisdom of the multi-use facility – only time will tell. The money is spent – at least it was put to better use than the ridiculous firehall.

  2. “…better use than the ridiculous firehall….” Our new firehall saved us several millions of dollars. It also put the fire hall on a major road which can save time – and lives – when responding to emergencies.

    We were under contractual obligation to the OPP to provide more space, proper change rooms/washrooms, lockers, a fully accessible facility (for both services, as per provincial law), etc. There was not enough space in the old building for both fire and police. One service was going to have to move.

    We had a choice: move the fire department to a new facility or move the OPP to a new one. Either way, we would have to rebuild the old building to meet provincial and contractual standards.

    We costed both options: the former was several millions of dollars less expensive than building a new police station. So we chose that direction.

    I never consider an option that achieves its goals AND saves tax dollars “ridiculous.”

  3. abbottaerospace

    If you were under a contractual obligation to the OPP to provide more amenities, that is a contract that I would revue. I live near the police station and am constantly astonished by the amount of police resources that we have available (and mostly inert) in our community.

    But lets say that the contract with the OPP was a good one and should be honored without question, and lets say that the best way to do that is to move the fire service to a new location – do they really need a Frank Lloyd Wright-esque architectural icon in order to serve Collingwood effectively? Could it not have been done at a significantly lower cost than constructing an impressive art-deco facility? I’m all for winning awards but we are the poor buggers that have to pay for it.

    I run Collingwood’s only state of the art aircraft development company (yes, you do have one here) and we work almost exclusively for US clients and I have to rent the top floor of a converted restaurant in a 150 year old building to keep my costs down – which I don’t mind, but I do mind when the excessive taxes that I pay as a corporation and individual fund this kind of grotesque folly. It also doesn’t help when our impressive firehall is occupied by vastly over paid unionized public employees that will enjoy a pension that I can only dream of (and also have to pay for). I don’t plan on retiring because there is no way, given what I compelled to pay for by our elected officials, that I will ever have the money to do it.

    It would be a good start when our elected official ward our tax money as carefully as if it was their own. Iconic public spending projects only throw into sharp relief how little the council really understand the practical world of real revenue generation and how carefully we have to spend out own money.

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