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• Introduction
• Deductive Reasoning
• Inductive Reasoning
• Examples of Deductive Reasoning
• Examples of Inductive Reasoning
• Valid, Invalid, Weak or Strong
• Conclusion and a Little Challenge

Introduction

How can it be that patients with brain lesions in the frontal lobe area often score reasonably well in IQ tests, but experience tremendous difficulty with real problems in their lives? If you ever found yourself acing those tests, but not doing so well with your daily problems, you might be surprised to learn that it might all be a matter of deductive versus inductive reasoning. If you’re not suffering of any brain lesion, like many things in life, this too can be changed as long as you’re committed to work on it.

Psychology and psychiatry scientists from three universities in US and Canada found that different regions of the brain are involved in deductive compared to inductive reasoning. IQ tests include both deductive and inductive reasoning problems. But the inductive items in these tests don’t usually simulate real world situations, rather they tend to be “improvised, involving artificial closed worlds”. Being able to deal with real issues in our lives means using inductive reasoning extensively, and not the artificial kind.

As far as the brain is concerned, it seems that when employing deductive reasoning, there is an activation in the part of the brain associated with language. The study is probably limited in this area, as this surely depends on the type of problems presented to the subjects. So if the problems were not mathematical, but language related, then this might explain the activation of that part of the brain.

By contrast, using inductive reasoning involves tasks including generalization and abstraction and these activate the medial region of the left prefrontal cortex.

But what does this mean for us? Well, let’s first take a step back and understand what deductive and inductive reasoning are.

Deductive Reasoning

Deductive reasoning is the process by which we come to a certain and specific logical conclusion starting from given general premises.

Another name for deductive reasoning is deductive logic. This process was first documented by Aristotle in the 4^th Century BC.

Here is an example of deductive reasoning:

All fish need water to survive (Premise 1)

My pet- Lulu is a fish (Premise 2)

Lulu needs water to survive (Conclusion)

There are two things which can help you recognize deductive reasoning:

1. 100% Certainty

An important characteristic of deductive reasoning is that the conclusion is 100% certain. If we find that the conclusion is incorrect, then we need to re-visit the premises and figure out which one is false. If Lulu does not need water to survive, maybe Lulu is not a fish? For deductive reasoning to work, all premises must be true.

2. The second characteristic has to do with the concepts of general and specific.

Let’s analyse these 3 sentences by pointing out if they are general or specific:

All fish need water to survive- General premise

My pet- Lulu is a fish- Specific premise

Lulu needs water to survive- Specific conclusion

So, in deductive reasoning, the approach General to Specific is top-down, meaning that we move from general to specific.

Inductive Reasoning

Inductive reasoning is the process by which we come to a probable conclusion starting from specific observations.

Inductive reasoning is also known as informal logic.

Historically, this method of reasoning came about much later than deductive logic, with early modern philosopher Francis Bacon who in 1620 stated that the natural world can only be uncovered by using processes of inductive reasoning.

Inductive reasoning is absolutely essential in today’s world. To learn about one of the most important models of argumentation, check our post on Toulmin’s Model for Argumentation.

Here is an example of inductive reasoning:

Bobo loves reading. (Observation 1)

Bobo is a good student. (Observation 2)

Good students love reading. (Conclusion)

You can probably already see some differences to deductive reasoning:

1. Probability: this conclusion that good students love reading is not certain. At most we can say that it is probable. We might find a couple of good students who do not love reading. Maybe they read because they know it is important.
2. Let’s also look at the phrases from general-specific perspective.

Bobo loves reading- Specific observation

Bobo is a good student- Specific observation

Good students love reading- General conclusion

In inductive reasoning, the approach Specific to General is bottom-up, from specific to general (see graphic above).

Examples of Deductive Reasoning

Here are some examples of deductive reasoning:

1

All planets are denser towards the middle.

Venus is a planet.

Venus is denser towards the middle.

2

Dogs bark.

My dachshund is a dog.

My dachshund barks.

3

All people are created equal.

My neighbour is a person.

My neighbour is equal to me.

4

All nouns are capitalized in German.

Substantive is a German noun.

Substantive is capitalized in German.

5

In Minecraft a village needs at least one house and at least one villager in order to be a “village”.

Enzo has a house and a villager in Minecraft.

Enzo has a village in Minecraft.

6

Nitwits cannot have their professions changed in Minecraft.

It is useless to try to change the profession of a nitwit in Minecraft.

7

Young leaves of dandelions taste good steamed, or tossed in salads.

Young leaves of dandelions are edible.

8

A cow is a female that has had at least one calf. A heifer is a female that has not yet had a calf.

Anna’s heifer has not had any calf yet.

9

The nuns in “Call the midwife” are Anglican.

Sister Evangelina’s was a nun in “Call the midwife”.

Sister Evangelina was Anglican.

10

Runners should eat complex carbohydrates 2 to 3 hours before a run.

Runners should eat oats 2 to 3 hours before a run.

11

Drinks high in refined sugar are bad for health.

Sugary soft drinks are bad for health.

12

All films shown in the UK need an age rating by law.

Captain Marvel needs an age rating to show in UK cinemas.

13

Chuck Norris can’t count to infinity.

14

The sphere has the smallest ratio of surface area to volume of any three-dimensional shape.

15

The speed of an apple reaching the ground depends on the height from which it fell.

Examples 13, 14 and 15 verbalize just the conclusions which have been reached through deductive reasoning. These are scientific conclusions with extensive proofs pre-determining them. Try to see if you can find the claims which support these.

Examples of Inductive Reasoning

Here are some examples of inductive reasoning:

1

A study covering 47 countries found that the higher a girl’s level of education, the more likely she was to express concern for the environment.

Educating girls improves entire communities.

2

June Collins has a hereditary allergy.

Noom Aibaju also has a hereditary allergy.

The tendency to develop allergies is often hereditary.

3

My father is Danish.

He has blonde hair.

All Danish people have blonde hair.

4

The first pencil I pull from my drawer is black.

The second pencil I pull from my drawer is black.

All pencils in my drawer are black.

5

My friend asks me to choose three books to lend her.

I choose 3 non-fiction self-development books.

All my books are non-fiction self-development.

6

Josie is afraid of dogs, cats and snakes.

Josie is afraid of all animals.

7

Vod doesn’t like literature or geology.

Vod doesn’t like any schools subject.

8

Tom’s guinea pig always sleeps after eating.

All guinea pigs sleep after eating.

9

The teachers in this high school are all young.

All high school teachers are young.

10

My Spanish friend has a funny accent in English.

All Spanish people have funny accents when speaking English.

11

There was an expensive black car in front of Minnie’s house this week on Tuesday and Friday.

Minnie has a rich friend.

12

My flowers in the back garden have been destroyed after last week’s storm.

All garden flowers are in danger during a storm.

13

Rain has destroyed this year’s harvest.

Rain is dangerous.

14

Lilly likes to draw pictures of her parents.

Lilly is 5 years old.

All 5 year old kids like to draw picture of their parents.

15

I come from a seaside town and I love swimming.

All people coming from seaside towns love swimming.

As you can tell, most of the inductive reasoning conclusions cannot be proven with certainty. Some of them are at most probable. Most of them need more data to support the truth of the conclusion.

Let’s try and flip them into deductive arguments.

For example number 6 can be flipped as follows:

Inductive

Josie is afraid of dogs, cats and snakes. (claim, specific)

Josie is afraid of all animals. (conclusion, general)

Deductive

Josie is afraid of all animals. (claim, general)

Josie is afraid of dogs, cats and snakes. (conclusion, specific)

Give it a try for yourself with some of the examples above!

Valid, Invalid, Strong or Weak

Deductive Reasoning- Valid or Invalid

Provided that the claim(s) is (are) true, in the deductive reasoning example we reach a conclusion which is 100% certain, thus we have a valid argument.

An invalid argument could be one where although the claims are true, the conclusion is false.

Consider this:

All Americans like pizza.

Joe likes noodles.

Joe is not an American.

In this example, although both premises are true, the conclusion makes an invalid assumption. In this case the conclusion is an overgeneralization.

Inductive Reasoning- Strong or Weak

By contrast the conclusion in the inductive reasoning example may or may not be true. So in the case of inductive reasoning it’s not a matter of valid or invalid arguments, rather a matter of strong or weak reasoning.

Conclusion and a Little Challenge

Let’s come back to the brain areas activated in different types of reasoning.

Inductive reasoning is an activity that is necessary for solving problems in everyday life or carrying out debates, while deductive reasoning is crucial in scientific demonstrations and discoveries.

None of these happen in a vacuum or on their own, we need both to advance as a species and the process of scientific exploration is one of re-iteration of these methods.

The aim is to become better at both. This will engage all part of the brain associated with these processes.

I leave you with a sample of an IQ test presenting an inductive problem.

Here is a logical sequence of five figures.

Which of the possible answers best matches the next figure in the sequence?