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What Is Deductive Reasoning? Learn the Definition of Deductive Reasoning With Examples, Plus 3 Types of Deductive Reasoning

Written by MasterClass

Aug 19, 2019 • 3 min read

There’s nothing better than deductive reasoning to win an argument or test a belief. But, while this type of logical argument produces rock-solid conclusions, not everyone can use it with certainty. Deductive arguments have to meet strict conditions. Knowing the ins and outs of deductive reasoning, and how to spot an invalid form of deduction, is a good way to sharpen your critical thinking skills.

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What Is Deductive Reasoning?

Deductive reasoning, or deductive logic, is a type of argument used in both academia and everyday life. Also known as deduction, the process involves following one or more factual statements (i.e. premises) through to their logical conclusion. In a deductive argument, if all the premises are true, and the terms correctly applied, then it holds that the conclusion will also be true.

This is alternatively referred to as “top-down” logic because it usually starts with a general statement and ends with a narrower, specific conclusion.

The general principles of deductive reasoning date back to the Ancient Greek philosopher Aristotle. Deductive reasoning is also at the heart of mathematics and computer programming.

What Is an Example of Deductive Reasoning?

Here is a simple example that illustrates basic deductive reasoning.

a) All men are mortal. (First premise)
b) Socrates is a man. (Second premise)
c) Therefore Socrates is mortal. (Conclusion)

Here, the conclusion is true because the second premise defines Socrates as part of the subset “men,” who the first premise stipulated are all mortal. Note the progression from a general statement to a specific conclusion.

What Is the Difference Between Valid Deductive Reasoning and Invalid Deductive Reasoning?

A “valid” deductive argument means that its conclusion must be true if its premises are true. But if the premises don’t necessitate the conclusion, the argument is “invalid.” For example:

a) All men are mortal. (First premise)
b) Lassie is mortal. (Second premise)
c) Therefore Lassie is a man. (Conclusion)

If there had been an additional premise that “only men are mortal,” then this conclusion would have been valid. As it is, Lassie can be a mortal dog.

3 Types of Deductive Reasoning

There are three common types of deductive reasoning:

  1. Syllogism
  2. Modus ponens
  3. Modus tollens

What Is a Syllogism?

One common type of deductive reasoning is known as a syllogism. Syllogisms almost always appear in the three-line form, with a common term that appears in both premises but not the conclusion. Here is an example:

a) If a person is born in the 1970s, they’re in Generation X.
b) If a person is in Generation X, then they listened to music on a Walkman.
c) Therefore if a person is born in the 1970s, then they listened to music on a Walkman.

What Is Modus Ponens?

Another type of deductive reasoning is known as modus ponens and it follows this pattern:

a) If a person is born between 1981 and 1996, then they’re a millennial.
b) Miley was born in 1992.
c) Therefore Miley is a millennial.

This type of reasoning is also known as “affirming the antecedent,” because only the first premise is a conditional statement, and the second premise merely affirms that the first part of the previous statement (the antecedent) applies.

What Is Modus Tollens?

Yet another type of deductive reasoning is modus tollens, or “the law of contrapositive.” It is the opposite of modus ponens because its second premise negates the second part (the consequent) of the previous conditional statement. For example:

a) If a person is born between 1981 and 1996, then they’re a millennial.
b) Bruce is not a millennial.
c) Therefore Bruce was not born between 1981 and 1996.

What Is the Difference Between Deductive Reasoning and Inductive Reasoning?

Deductive reasoning is one of two basic types of reasoning that feature in a logical argument. The other is inductive reasoning. Where deductive reasoning is top-down thinking, an inductive argument is bottom-up—it starts with specific premises and draws a general conclusion from them.

For example:

a) Miley and Jonas are millennials.
b) Miley and Jonas live in rented housing.
c) Therefore all millennials live in rented housing.

While this seems like it might produce some false conclusions—and sometimes it does—inductive reasoning plays an important role in problem-solving and decision making. It is about arriving at probably rather than certain conclusions. It is often how we develop theories or hypotheses about the world, and we can go on to test these by accumulating more data.

Learn more about inductive reasoning here.

What Is the Difference Between Deductive Reasoning and Abductive Reasoning?

Abductive reasoning starts with specific observations and seeks the most likely explanation for them. It is the equivalent of the best guess. It can’t produce a definitely true conclusion like deductive reasoning, but it can still be a helpful way to process the real world. For instance:

a) Miley and Jonas are millennials.
b) Miley and Jonas live in rented housing.
c) Therefore many millennials can’t afford to buy their own homes.

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