There are a number of ways an argument can be fall flat, sometimes called logical fallacies. These would make an argument not valid, even if the premise is correct. You don’t need to remember the names but it might be useful to think about some of the following, which crop up more than you might think.

 

 

Non-sequitur

This is the most frequent fallacy. The argument is not valid because one logical step does not lead to the other. It sounds like it should be obvious and sometimes it is but it is always worth thinking whether the conclusion necessarily follows from the argument presented.

"Jane can't play football because she is a girl." This argument is presented in a logical fashion and so may stand up to cursory examination but it should be apparent fairly quickly that there is no logical conection between being a girl and not being able to play football. It is a non-sequitur.

 

Straw man

This is where you explain a really weak version of your opponent’s view (set up a straw man) and then explain where it is wrong. It can be very effective at persuading people but the fact that a poor version of the argument is not true, doesn’t mean the proper version isn’t.

 

http://www.youtube.com/watch?v=BzC135ql_wA&t=6m30s

In this clip from President Obama’s second inaugural address he claims:

“No single person can train all the math and science teachers we’ll need to equip our children for the future, or build the roads and networks and research labs that will bring new jobs and businesses to our shores”

The inference being that therefore the government should be involved. Whether or not he is correct that the government should provide those things, the argument is not valid – nobody is arguing that one person can do those things, although some claim that large groups of private individuals can do some of those things without government assistance. He has set up a straw man that is easy to demolish.

 

Correlation equals causation

Just because two things happen together or one after the other does not mean one thing causes the other. We are used to the idea that, for example, the sun goes behind a cloud and we get cold. We assume that the sun going in causes us to get cold because it happened first. In this case we would be right but consider mobile phone masts. For each mobile phone mast in a given area, the number of babies born rise by 17.6. The research is robust and the result correct. We might conclude that mobile phone masts are causing the births. In reality the cause is much simpler. People have babies and mobile phone masts are put where people are. One does not cause the other, there is a third factor influencing them both.

This argument is sometimes referred to as "post hoc ergo propter hoc" (Latin for afterwards therefore because of). Just because one event happens after another (they appear correlated) does not mean that one causes the other. The events may have happened by chance or there may be an additional unknown factor influencing both.

 

https://youtu.be/HL_vHDjG5Wk

 

Ad hominem/arguments from authority

These are two different fallacies but different sides of the same coin. Ad Hominem arguments are when rather than arguing against the opponent’s argument a person argues that their opponent is not competent or not believable. An argument from authority is the opposite. A person argues they are correct because of their experience or the experience of someone who agrees with them. While this experience may lend credibility to their assumptions but it does not make their argument valid.

An example might be a Conservative politician arguing that the policies Labour are arguing for are wrong because of their record on the economy. It is possible that Labour made mistakes in the past, they may even be as incompetent as some would like to make out but it does not follow that their argument is invalid on this occasion.

  

What next? 

This is just a flavour of some of the logical fallacies you might come across. If you are interested in reading more, there is plenty of information available.  You could start with the site https://yourlogicalfallacyis.com where you can find a larger number of fallacies to explore or you you could try reading “The Duck that Won the Lottery” by Julian Baggini