5 Logical Fallacies Worth Recognising

Logical fallacies are the verbal equivalent of optical illusions; flaws in reasoning that sound convincing, but do not hold up under scrutiny.



In debating, spotting a logical fallacy in the opposition’s argument makes for easy rebuttal. There are five logical fallacies beginning debaters are particularly likely to fall back on.


1. Slippery Slope If we allow A to happen, then eventually Z will happen, so we should not allow A in the first place.

This is usually presented as a hypothetical situation, the outcome of which is taken to extremes. For example: if school sports were optional, some people would elect not to do any sports. They’d just lie around on the couch all day, getting fat, until their bones atrophy and they can’t walk anymore.

Why it gets used: The debater wants to impress on the audience the importance of what they’re saying, so they exaggerate the possible consequences.

How it gets heard: The audience does not buy into the unrealistic outcome, and consequently dismisses the argument as fear-mongering.

How to avoid it: Stick to real-world examples rather than hypothetical situations and remember that outcomes can be undesirable without being the end of the world.

2. False Dilemma Presenting two alternatives as the only possibilities.

This is also called ‘black-or-white’ or ‘binary’ thinking. The debater presents two possibilities – the argument they’re championing, and a clearly undesirable alternative – and asks the audience which of the two they’d prefer, as though it has to be one or the other. For example: if we don’t allow the police to use tasers they’d have to rely on guns – and nobody wants the police more reliant on guns, do they?

Why it gets used: It’s an easy mode of thought to default to, because it requires active imagination to come up with other alternatives. It’s especially common in model topics (topics that propose a change to the world, usually using the word ‘should’), where the two alternatives are the status quo and the proposed model.


How it gets heard: Unfortunately, this is often heard as a logical argument. The audience doesn’t have time to imagine other options, so as long as both alternatives sound realistic, the false dilemma is often accepted.


How to avoid it: Be on the lookout for anything framed as an either/or. The world is always more complicated than that!


3. Anecdotal


Using a personal experience or isolated example instead of a sound argument or compelling evidence.


For example: My grandfather smoked around 30 cigarettes a day and lived until 97, so smoking isn’t as dangerous as people think.


Why it gets used: It’s much easier to come up with an anecdote than it is to do the research and understand what’s statistically likely.


How it gets heard: Human beings like human connections, and our inclination is to believe personal perceptions and experiences over abstract data. As long as the story is relevant to the point being made, anecdotes are often accepted as evidence. They’re more convincing with grounding details that ‘prove’ they really happened, like names and dates as opposed to “I heard about this guy who…”


How to avoid it: Remember that real life usually falls along a bell curve. There will be extraordinary examples at either end of that curve, but those atypical instances are not representative. Whenever possible, trust methodologically sound studies over individual examples – if something has been published in a peer-reviewed journal, that’s usually a pretty good sign!


4. Generic


Judging something as good or bad based on where or from whom it comes.


Good people can say or do bad or incorrect things, just as bad people can say or do good or correct things. How you feel about a person has nothing to do with whether their argument has merit. For example: Hitler was a vegetarian, a fact that says absolutely nothing about the morality of any dietary choices.


Why it gets used: We are inclined to trust things said by people we believe to be trustworthy; “If X says it, it must be true.” We are even more disinclined to be similar to, or believe similar things, to someone we find personally repugnant.


How it gets heard: This one gets easier to pick up the more frequently a particular example is used. Hitler and Gandhi have been overused to the point that most audiences easily dismiss the comparison, but less commonly used examples often go unquestioned.


How to avoid it: A quote makes a good hook or closing thought, but it is not an argument. Always make sure the argument is supported by evidence, rather than being carried by the weight of reputation.

5. Appeal to authority An authority thinks something, therefore it must be true.


Appeals to authority are valid when the authority is an expert in the field being discussed, although it is important to remember than even experts sometimes get it wrong. The problem is when the aura of authority is used to lend credence to something about which the individual in question is not an expert. For example: this brand of cereal is the best, because it’s what athlete Michael Jordan eats every day for breakfast.


Why it gets used: Deciphering actual data and experimental results is difficult for people not in the relevant field, and virtually impossible for beginning debaters. We must rely on experts to translate their work into terms a layperson can understand. Even then, it can be difficult to determine whether a given source is relevant and reliable.


How it gets heard: While an incredibly insidious logical fallacy in real-world situations, the appeals to authority made by beginning debaters tend not to be convincing. Audiences find appeals to authority persuasive only when the authority’s credentials are clearly established, which is a step beginning debaters tend to miss.


How to avoid it: A source should always be referenced with ‘gloss’; that is, a description of who or what that source is, and why their opinion matters. If there’s no gloss, or if the gloss doesn’t seem relevant, don’t treat them as an expert. Bill Nye is a scientist, which might make him sound like a reliable source for scientific facts, but his field of study is mechanical engineering; he is not an expert when talking about climate change. By recognising these logical fallacies, debaters can avoid using them in their debates, as well as offer more effective rebuttal by spotting them in their opposition's arguments.