Two sorts of reasoning: deductive vs. inductive reasoning

Previously we examined the issue of whether or not humans are rational creatures. The answer turned out to be a little complicated. If we accept that the rules of logic tell us how to decide whether an argument is valid or invalid then we also have evidence that shows that humans do not always reason along the same lines. Whereas some logical arguments are readily understood others are not. Up until now the material we have been discussing concerns something known as deductive reasoning. Deductive reasoning is broadly defined as thinking by following the rules of logic. Even though humans don’t appear to be natural logicians, we still seem to be doing alright. So what sort of reasoning do humans more readily engage with?

And this is where we turn towards what is known as inductive reasoning. Whereas deductive reasoning deals with certainties and necessities. Inductive reasoning deals possibilities and likelihoods. So if there is a 90% chance of rain whilst I am out at the shops should I take my umbrella? In the same way we that we have made much progress in understanding the nature of human reasoning by studying deductive reasoning, we made similar progress by studying inductive reasoning. We illustrate some of what cognitive psychologists have found out about inductive reasoning and as with deductive reasoning there is both good news and bad. Lets assume that you move to the Caribbean for an extended holiday and arrive on the 1st November.

Over the following days you notice that sometimes in the morning a very distinctive cloud formation appears in the sky. If these clouds appear in the morning then this is a really good indication that later in the day there will be a rain storm. Very soon you conclude that the distinctive clouds in the morning are storm clouds and so begin to pack an umbrella in anticipation of the afternoon rain. So what has gone on here Well you have made a limited number of observations of the appearance of certain clouds in the morning and from these observations you conclude that these clouds appear to predict the occurrence of rain in the afternoon.

Now in arriving at this conclusion you have not applied a rule of logic you have merely observed how the world is and you have drawn an inference. From making a small number of observations you have then drawn a general inference. If in the future you see the same sort of clouds appear in the morning you conclude that the chances of rain in the afternoon are high and so pack the umbrella. You cannot be certain that rain will come but your previous observations lead you to predict that it probably will. You have reasoned inductively. To reiterate inductive reasoning deals with uncertainties and likelihoods so whereas the inferences we draw work most of the time some of the time they fail.

So if we are to reason inductively then we must expect errors to occur – we draw the wrong conclusion and either fail to take the umbrella when it does rain or take it when it doesn’t. You may end up deciding that it is these kinds of failures that reveal more about human reasoning than the successes.