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Skip to 0 minutes and 15 secondsWhen you're faced with an argument in the wild, here's a diagram that will help you evaluate it. The first step is to identify the main argument and to put it in standard form. And remember to be charitable. Once you have your standard form, you have to decide if the argument is to be evaluated as being deductive or non-deductive. A deductive argument is one that is offered to provide logically conclusive support for its conclusion. A non-deductive argument is one that is offered to provide probable support for its conclusions, but not conclusive. The rule of thumb is to treat arguments as being non-deductive unless the intention is clearly deductive. Now, suppose that you're facing a deductive argument.

Skip to 1 minute and 3 secondsYour first task is to evaluate if the argument succeeds logically. That is, you want to know if the argument is valid because it's a deductive argument. If the argument is invalid, game over. The argument is bad and you're done. And the best way to show that an argument is invalid is by providing a counter-example, a situation in which the premises are true and the conclusion false. If the argument is valid, you need to proceed to the next step and see if it is sound. If all premises are true, then you have a sound argument. A sound argument is as good as it gets.

Skip to 1 minute and 42 secondsThe premises give conclusive, logical support for the conclusion and they are all true, which means that the conclusion is also true. As we've discussed before, some of the premises might be backed up by sub-arguments. If that's the case, you need to suspend the evaluation of the main argument and evaluate each sub-argument individually. Suppose you have a sub-argument for a premise and you conclude that this sub-argument is bad. Then that premise has lost its support and the argument collapses. Game over-- the argument is bad. But if all premises are true or are backed up by a good argument, then you have a sound argument and you can conclude that it is good.

Skip to 2 minutes and 29 secondsIf you have a non-deductive argument, the procedure is pretty much the same. You first need to decide if the premises provide the required logical support. But non-deductive arguments are tricky. What makes an argument strong might depend on the context of evaluation. If you are in a court of law, you want arguments to be very strong. Otherwise, you might put too many innocent people in jail. But if you're at a party with your friends and family, you might want to adopt looser standards. To show that an argument is weak, you need to give a counter-example. But not any counter-example will do. You need to find a credible scenario in which the premises are true and the conclusion false.

Skip to 3 minutes and 14 secondsIf you can't do that, then the argument is strong and you move on to inspect the truth of the premises. If all premises are true or backed up by good sub-arguments. Then the argument is cogent and therefore good. Otherwise, it's bad. How do you know if a premise is backed up by a good sub-argument? Simple-- isolate the sub-argument and use the diagram to evaluate it. And repeat for as many sub-arguments as there are. And that's how you evaluate arguments as a good critical thinker.

How to evaluate an argument

This video shows you how to evaluate arguments in a step-by-step manner:

  1. Identify the conclusion and the premises.
  2. Put the argument in standard form.
  3. Decide if the argument is deductive or non-deductive.
  4. Determine whether the argument succeeds logically.
  5. If the argument succeeds logically, assess whether the premises are true. For premises that are backed-up by a sub-arguments, repeat all the steps for the sub-arguments.
  6. Make a final judgement: is the argument good or bad?

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This video is from the free online course:

Logical and Critical Thinking

The University of Auckland

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