What is Deductive Reasoning? Definition and Examples

Deductive Reasoning Definition and Examples

First, let’s define deductive reasoning

Deductive reasoning, or deductive logic, is used to determine whether premises add up to a sensible conclusion. But for a conclusion to be made, deductions must be tested. We’ll get into some deductive reasoning examples but let’s start with a definition.

For example, say an angry consortium of villagers intend to burn a witch. They say that since witches float in water, they must be as light as a duck. To test this, the villagers weigh the witch against a duck.

The logic suggests that if the witch and the duck are the same weight, then the woman must be a witch. Sound familiar? That’s because it’s taken from a scene from Monty Python and the Holy Grail. Check out the clip below:

This scene satirizes many of the issues that exist with deductive and inductive reasoning — but it also shows that you can expose these follies for comedic effect. So what is deductive reasoning used for? 

Inductive and Deductive Thinking

Inductive vs deductive reasoning

I know what you’re thinking, “what’s the difference between inductive and deductive reasoning?” Good question! Deductive and inductive reasoning are opposites — deduction applies a top-to-bottom (general to specific) approach to reasoning whereas induction applies a bottom-to-top (specific to general) approach. This next video explores in further detail the ways in which examples of the two modes are different.

Here’s another way to think about inductive vs deductive reasoning:

Deductive reasoning starts with a general assumption, it applies logic, then it tests that logic to reach a conclusion. With this type of reasoning, if the premises are true, then the conclusion must be true.

Logically Sound Deductive Reasoning Examples:

• All dogs have ears; golden retrievers are dogs, therefore they have ears.
• All racing cars must go over 80MPH; the Dodge Charger is a racing car, therefore it can go over 80MPH.
• Christmas is always Dec. 25th; today is Dec. 25th, therefore it’s Christmas.

Logically Unsound Deductive Reasoning Examples:

• All zebras have stripes; tigers have stripes, therefore tigers are zebras.
• Fourth of July always has fireworks; today there were fireworks, therefore it must be the Fourth of July.
• Carrots are orange; oranges are orange, therefore oranges are carrots.

Inductive reasoning starts with a specific assumption, then it broadens in scope until it reaches a generalized conclusion. With inductive reasoning, the conclusion may be false even if the premises are true.

Generalized Inductive Reasoning Example:

• There are a total of 20 apples and oranges in a basket. I pulled out five; four apples and one orange, therefore there are 16 apples and four oranges in the basket.

Predictive Inductive Reasoning Example:

• Most baseball players become coaches. Eduardo is a baseball player, so he’ll become a coach.

Statistical Syllogism Inductive Reasoning Example:

• 95% of Oxford graduates went on to get PhD’s; Rocky graduated from Oxford, therefore he’s going to get a PhD.

There’s a third type of reasoning called abduction. Abductive reasoning is a predictive inference in which we guess the most likely conclusion given a specific set of premises. Here’s an example:

• I arrived home to find the birthday cake crudely eaten. Nobody was home besides my dog. My dog must have eaten the birthday cake.

Deductive Reasoning Examples in Everyday Life

Types of deductive reasoning

There are three major types of deductive reasoning we can use to test deductions: syllogism, modus ponens, and modus tollens. Let's break these down one at a time.

Syllogism

Syllogism is probably the most simple of the 3 types of deductive reasoning. In simplest terms syllogism states that if A=B and B=C, then A=C. It takes two separate clauses and connects them together. A more creative example would be: a puma is a cat, cats are mammals, therefore pumas are mammals.

Modus Ponens

A modus ponens is when a deduction is presented as a conditional statement, proven by subsequent clauses: the antecedent and consequent. For example: Every player on the Boston Celtics is between the ages of 21 and 31. Jayson Tatum is on the Boston Celtics, therefore he must be between 21 and 31.

Modus Tollens

A modus tollens is the opposite of a modus ponens. Whereas the latter affirms a conditional statement, the former refutes it. For example: The freezing point of water is 32 degrees Fahrenheit. It’s hotter than 32 degrees Fahrenheit, so water will not freeze. 

Deductive Reasoning Meaning

Deductive reasoning in storytelling

In storytelling, deductive reasoning means more than what it was intended to. This type of reasoning has become a carte blanche term for when a character makes any sort of inference. A lot of the time, we say that inferences are deductions when in reality, they’re inductions or abductions.

Sherlock Holmes is a character that appropriately applies deductive thinking to reach logical conclusions. By this, I mean that when Sherlock makes an inference, the premises of the assumption usually add up to the conclusion. Let’s take a look at a clip from the BBC series Sherlock to see how deductive reasoning is used in TV:

It may be a little difficult to hear Sherlock explain deductive reasoning here due to the breakneck speed in which he speaks, but if you listen closely, you’ll hear that all the premises add up to a logical conclusion.

If the inferences were inductive, Sherlock would acknowledge a generalization. If they were abductive, he’d have to suggest probability. But he does neither. Instead, he promises that his inferences are true, and that they must be true because all premises are accounted for.

So what is deductive reasoning? Well, it’s fair to say the deductive reasoning method is used to test the premises of a statement. When used correctly, deductions allow us to see the world in mostly objective sense.

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