Rhetological Fallacies (errors and manipulations of rhetoric and logical thinking) by David McCandless.
The most senior Catholic Cardinal here in the UK recently outlined his argument against same-sex marriage. Here’s our rhetological matrix applied to his speech.
When Looking At Arguments:
1) Are the reasons true?
2) Does the conclusion/s follow from those reasons – is the inference good?
If: 1 and 2 = cogent / sound. Which means we can be convinced by it.
NB: arguments are not true or false – they are only sound or unsound [as ‘un-cogent’ isn't a word!]
Cogency/soundness = overall reasoning
Validity – is only for inferences, and we can see this in different degrees:
Weak validity – Slightly likely. Inferences are unacceptable, but at least the reasons are relevant to the conclusion. In a weak argument, even if the reasons were true, the conclusion would only be slightly more likely as a result. ‘The rooster crowed yesterday, it rained yesterday, therefore it will rain today because the rooster crowed today’. [Post hoc ergo propter hoc fallacy!]
Moderate validity – Fairly likely. If the reasons are true, then it is fairly likely (although not inevitable) that the conclusion will be true too. Some support, but not strong support. ‘Australia did well at the last Olympics, therefore they’ll do well at the next Olympics’.
Strong validity – very likely. Where the reasons are almost conclusive. ‘The price of wool is falling, while the demand for wool is rising – therefore wool prices will start to increase’.
Deductively valid – must necessarily follow. Where they must necessarily follow. ‘He has often lied to me, therefore he is not a completely honest person.’
Slideshare notes - summary of validity, truth, soundness, strength and cogency
The Validity and Soundness of Deductive Arguments – includes exercises
Tutorial on Validity - includes exercises
Validity and Soundness – Practice Exercises
It’s Saturday, so I’m taking a break – but I think you should check out the following poster:
There’s a number of podcasts that have investigated logical fallacies – here’s a few that you might like to check out:
And the InFact Vodcast:
A ‘straw man’ argument is one in which the opposing arguer has deliberately been made the original argument weak to the point where no one would be likely to support it. It gets its strange name from the custom of making human figures out of straw for target practice.
A straw-man argument works like this:
Another way of putting it is:
When creating arguments, it is possible to produce fallacies. Generally a fallacy is considered to be a a mistake, a bad argument or even a deceptive or misleading one. It would be more accurate to consider a fallacy to be a flaw in the argument that makes it invalid or unsound.
When we talk about soundness (or cogency), we mean that an argument must be both:
- all of its premises are true.
Soundness / cogency has also been mentioned in the videos previously featured on the site, so you can review Week Two as well.
For instance, a classic example is:
All men are mortal.
Socrates is a man.
Therefore, Socrates is mortal.
The argument is valid (as the conclusion is true based on the premises – that is, that the conclusion follows the premises) and
since the premises are in fact true, the argument is therefore sound / cogent.
Here’s an example of an invalid argument:
All men are mortal.
Socrates is a man.
All men are Socrates.
While the premises are true, the conclusion doesn’t follow – making it invalid (and it’s also a fallacy called a non sequitur, as the conclusion does not follow logically from the premisses used to support it).
Here’s another example – the following argument is also an example of an invalid argument (and it’s a cartoon I featured previously on the site):
Penguins are black and white
Some old tv shows are black and white
Therefore, some penguins are old tv shows
For the rest of the week, I’ll go through a number of logical fallacies, including videos and audio to help revise.