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Skip to 0 minutes and 14 secondsI hope you've enjoyed my discussion with John Bishop on the distinction between deductive and non-deductive arguments. It illustrates well how the critical thinking skills you're learning in this class are central to debates and philosophy. Of course, the scales are not only useful when you think about deep issues like the existence of God. You need the same skills for any kind of argument you encounter on a daily basis. Let me emphasise once more how important the principle of charity is for a critical thinker. To be charitable is to treat others as intelligent. If you're charitable, you'll do a much better job at evaluating arguments. And this is most important if you don't agree with someone's view.

Skip to 0 minutes and 58 secondsYou'll do a much better job if you can show that the best version of their argument fails. It might be easier to show that an argument is bad when you're not charitable, but then your evaluation is worthless. Charity is also important when deciding if arguments are deductive or non-deductive. As a rule of thumb, treat argument as being non-deductive, unless the intention is clearly deductive. Most people don't know the distinction between the two and they would benefit from a charitable reading of their arguments as being non-deductive. The goal is not to prove them wrong but to find the best arguments. This concludes your first introduction to logic. Next week, we'll cover the remaining material needed to evaluate arguments.

Skip to 1 minute and 44 secondsBasically, if you have an argument that succeeds logically because it is a valid deductive argument or a strong non-deductive argument, your next step will be to inquire about the truth of the premises. This week, we didn't care if premises were false and concluded that rubbish arguments are valid, like this one, Premise 1, if the moon is made of blue cheese, then I'm a zebra. Premise 2, the moon is made of blue cheese therefore, conclusion, I'm a zebra. The argument is valid, but it's rubbish. However what makes it a bad argument is not a logical mistake, right? See you next week.

Summary of Week 3

So what have we done this week?

We’ve introduced you to the principle of charity: to be charitable is to treat others as intelligent. This is most important if you don’t agree with someone’s view. You’ll do a much better job if you can show that the best version of their argument fails.

We’ve talked about the important distinction between deductive and non-deductive arguments. How do you choose whether an argument is deductive or non-deductive? Simple: unless the argument clearly indicates that it is deductive, apply the principle of charity and take it to be non-deductive.

We also learnt to decide when arguments succeed logically.

  • A deductive argument succeeds logically if it is valid: it is impossible for the premises to be true and the conclusion false.

  • A non-deductive argument succeeds logically if it is strong: it is improbable for the premises of the argument to be true and the conclusion false.

Of course, for an argument to succeed logically doesn’t mean that it’s a good argument. It’s all good to know that it’s impossible (or improbable) for the premises of the argument to be true while the conclusion is false, but are the premises actually true? This is the next stage in argument evaluation: once you know that argument succeeds logically, then you go ahead and ask if the premises are true. That’s what we’ll focus on next week.

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Logical and Critical Thinking

The University of Auckland

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