Posts from the “Fallacies” Category

January 20th – Summary of Evaluating the Inferential Strength of Argument

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!]

  • Reasons and conclusions can be true or false
  • Inferences can be valid or invalid


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 validityvery 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 validmust necessarily follow. Where they must necessarily follow. ‘He has often lied to me, therefore he is not a completely honest person.’

Additional Links:

Slideshare notes summary of validity, truth, soundness, strength and cogency

PDF – Testing Arguments for Validity and Soundness

The Validity and Soundness of Deductive Arguments – includes exercises

Tutorial on Validity – includes exercises

Validity and Soundness – Practice Exercises

Truth, Validity and Soundness Quiz

January 18th – Learning More About Fallacies Via Podcasts

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:

January 15th – Straw Man Fallacy


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:

  1. An argument is presented,
  2. The argument is then simplified and distorted by the opposing arguer
  3. He will then base his counter-argument on the distorted version, which is easy to knock down – he is arguing against an opponent who doesn’t really exist.
  4. It looks as if he has successfully demolished the original argument. He has done nothing of the sort.

Another way of putting it is:

Read more

January 14th – Introduction To Fallacies

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:

– valid.
– 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.