Critical thinking

Alfred Korzybski wrote:
There are two ways to slide easily through life: to believe everything or to doubt everything. Both ways save us from thinking.”

The Watson-Glaser Critical Thinking Appraisal is a famous critical thinking test. Watson and Glaser define critical thinking as a combination of:

An attitude to inquiry that involves the ability to recognise problems and an acceptance of the need for evidence in support of claims.

Knowledge of the rules and restrictions of logical reasoning.

The skill to apply this attitude and knowledge.


So how do we do this then? First, we take the attitude that nothing is going to get past us. We are the bouncers on the door of debAte, the members’ club where all the coolest arguments go. If they’re not on the list, they’re not coming in. How do arguments get on the list? They have to satisfy these criteria:


  • All claims are backed up with reasoning or evidence or both.
  • The conclusions follow logically from the premises (i.e. the argument is valid).
  • The premises are true (provably, or at least there is evidence for them being true).
  • There are no redundant or stuck-on conclusions.


If you make a claim, whether factual or emotional, you need to back it up. The most common mistake in written argument is making unjustified claims. You can justify your claim with reasoning or with facts, but using both is better.

The sky is blue.
This is an unjustified claim.

The sky is blue. This is clear from the fact that when we look up at the sky, we see blue.
This is a reasoned claim, but has no references. The reasoning is flawed, so references might have saved the argument.

The sky is blue because of ‘Rayleigh scattering’. As light moves through the atmosphere, most of the longer wavelengths (red end) pass through. However, much of the shorter wavelength light (blue end) is absorbed by gas molecules. This absorbed blue light is then radiated in every direction, getting scattered all across the sky. Whichever direction you look, some of this scattered blue light reaches you. So you see a blue sky. (See Fraser 1987; Gerrard & Bernstein 2000; and Graham 1923.)
This is a reasoned claim that has three separate references. This is a Very Good Argument.

This is also something to look for in other people’s arguments; seeing whether someone has backed up their claim will help you to evaluate how strong their argument is.

There is a detailed tutorial on the logical mistakes that can occur in arguments on this blog (How to Win an Argument, parts 1-4). However, if you don’t want to read 7,000 words on logical fallacies, here is a short introduction to how logic can help you think critically:

Non sequitur is Latin for ‘doesn’t follow’. This describes an argument where what someone claims doesn’t follow from what they started with. This might be because the argument is invalid or unsound (we’ll look at ‘soundness’ in the next section).

Most arguments say something like “because of A and B, therefore C”. A and B are called premises and C is called the conclusion. The ‘validity’ of an argument is just based on its structure, the ‘soundness’ is based on whether the statements are really true.


If the premises are true, then the conclusion is true.


If the premises are true, then that doesn’t necessarily mean the conclusion is true (it could be true, but that would just be a coincidence).

Valid argument 1

All men are mortal.

Ben is a man.

Therefore, Ben is mortal.

Valid argument 2

All animals with wings can fly.

Penguins have wings.

Therefore, penguins can fly.

The second argument is definitely valid, but its conclusion is false – penguins can’t fly. How has this happened? The first premise “All animals with wings can fly” is false.


Notice that ‘validity’ is only about the logical structure of the argument, not about whether any of the statements are really true. The first argument above is sound, the second one – because its first premise is false – is unsound.

So, we’ve seen how even valid arguments can produce false conclusions – because they are unsound:


A valid argument with true premises (and so a true conclusion)


A valid argument with at least one false premise.
An invalid argument.


(If an argument is invalid, it is automatically unsound.)


We could argue that “All animals with wings can fly” is actually true. This would stop the argument being unsound and so mean that is was a good argument. To do this, we would need to provide evidence or reasoning (see point (1)) to show that it was true.

We would only need to show one animal with wings that could not fly to show that the whole argument is unsound. A penguin, for example.

As well as using things that are wrong to show that an argument is unsound, you can use unsound arguments to prove that things are wrong. This is called reductio ad absurdum, which means ‘reducing to an absurdity’. For example, if your opponent claims that “All animals with wings can fly”, then you can use the argument above to conclude that penguins can fly (which is absurd) and so show that your opponent’s claim is false.

All of these four criteria for good arguments can be difficult to spot or difficult to use. Redundant or stuck-on conclusions are probably the most difficult.

Imagine an argument is trying to get into the members’ club debAte. He’s got good credentials, this argument, but he’s got a few friends with him. If we let him in, should we let his friends in without checking their credentials? No. If an argument appears to conclude two different things (or more), then we need to check each conclusion separately.

Here is an extreme example to illustrate: Consider that “1+1=2” and “2+2=4” are true statements.

We could say something like:

If “1+1=2”, then “2+2=4”
Therefore, 2+2=4

This argument is valid and sound. But because “2+2=4” is always true, that means that this next argument is also valid and sound:

If “1+1=2”, then “2+2=4 or the moon is made of cheese”
Therefore, 2+2=4 or the moon is made of cheese

So we have managed to conclude that the moon being made of cheese is somehow related to 1+1=2 and might actually be true!

Keep a watchful eye out for these stuck-on conclusions; most of the time they are not even known to the person giving the argument.

So, a good argument needs to satisfy the entry conditions:

  • All claims are backed up with reasoning or evidence or both.
  • The conclusions follow logically from the premises (i.e. the argument is valid).
  • The premises are true (provably, or at least there is evidence for them being true).
  • There are no redundant or stuck-on conclusions.

Article by Will Bentinck originally for Debatewise but still applicable to debatabase and more generally


Syndicate content