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Skip to 0 minutes and 7 seconds We are now familiar with the important distinction that has been drawn between deductive and inductive reasoning. Deductive reasoning is characterised by following the rules of logic whereas inductive reasoning is more concerned with drawing reasonable conclusions based on limited evidence. We have considered some of the evidence that shows how well humans reason accordingly. Over the years there have been many cognitive accounts of the particularities of human reasoning and a family of theories has emerged that draw a slightly different distinction. Recently this has been characterised as the difference between thinking fast and thinking slow. In fact theorists have even gone further and argued that humans possess two different reasoning systems Think in terms of smart phone apps.

Skip to 1 minute and 0 seconds There is one standard txt app that comes with every phone and then there is WhatsApp. The two apps do more or less the same thing – send and receive txt – but they are different and have different pros and cons. We might therefore think of two different cognitive apps for thinking We will call these System 1 and System 2 respectively. System 1 – is fast and intuitive System 2 – is slower and more mentally demanding. Now we might think that System 1 does inductive reasoning and System 2 does deductive reasoning but this is not the kind of picture that has emerged.

Skip to 1 minute and 41 seconds For instance remember mortal men Harry being a man and therefore being mortal – well most people draw that conclusion very quickly. So what kind of evidence has been used to support the idea of two reasoning systems? Let’s begin with the following example and answer as soon as you can. A bat and ball cost £1.10. The bat costs £1 more than the ball. How much does the ball cost? If you answered 10pence then this reflects the intuitive knee-jerk response to the question that System 1 produces. This is the wrong answer. If you did answer incorrectly then reconsider and come up with the right answer. Now you have more time and can generate a more considered response.

Skip to 2 minutes and 31 seconds It is now that you have engaged the System 2 app – it is responsible for the slower more considered thinking. According to such dual process accounts, there are two systems of reasoning. Each has its own characteristics and the influence of each can be seen depending on the testing circumstances. Under time pressure responses will most likely reflect the operation System 1 the fast intuitive system. When there is no time pressure and where the consequences of making an error are high then responses reflect the operation of System 2 – the slower more careful system. Despite this characterisation it would be mistake to argue that errors in reasoning reflect solely the operation of System 1 – more haste less speed!

Skip to 3 minutes and 23 seconds Errors in reasoning can arise even when System 2 has all the time in the world. If we accept this however we need to be clearer on what we mean by an error. One form of reasoning error is where we draw an invalid conclusion from a claim and the evidence. However, errors may also arise when our conclusions contradict the actual facts of the world. It is these sorts of error that we now need to consider in more detail.

Two modes of thinking: fast vs. slow

Here Rob introduces you to the notion of two modes of reasoning that seem to suggest the existence of two very different kinds of reasoning systems.

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Introduction to Cognitive Psychology: An Experimental Science

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