# Deductive reasoning

# Deductive reasoning

**Deductive reasoning**, also **deductive logic**, is the process of reasoning from one or more statements (premises) to reach a logically certain conclusion.^{[1]}

Deductive reasoning goes in the same direction as that of the conditionals, and links premises with conclusions. If all premises are true, the terms are clear, and the rules of deductive logic are followed, then the conclusion reached is necessarily true.

Deductive reasoning (*"top-down logic"*) contrasts with inductive reasoning (*"bottom-up logic"*) in the following way; in deductive reasoning, a conclusion is reached reductively by applying general rules which hold over the entirety of a closed domain of discourse, narrowing the range under consideration until *only* the conclusion(s) is left. In inductive reasoning, the conclusion is reached by generalizing or extra expolating from specific cases to general rules, i.e., there is epistemic uncertainty. However, the inductive reasoning mentioned here is not the same as induction used in mathematical proofs – mathematical induction is actually a form of deductive reasoning.

Deductive reasoning differs from abductive reasoning by the direction of the reasoning relative to the conditionals. Deductive reasoning goes in the same direction as that of the conditionals, whereas abductive reasoning goes in the opposite direction to that of the conditionals.

Simple example

An example of an argument using deductive reasoning:

All men are mortal.

(First premise)

Socrates is a man.

(Second premise)

Therefore, Socrates is mortal.

(Conclusion)

The first premise states that all objects classified as "men" have the attribute "mortal."

The second premise states that "Socrates" is classified as a "man" – a member of the set "men."

The conclusion then states that "Socrates" must be "mortal" because he inherits this attribute from his classification as a "man."

Reasoning with modus ponens, modus tollens, and the law of syllogism

Modus ponens

(First premise is a conditional statement)

(Second premise is the antecedent)

(Conclusion deduced is the consequent)

The following is an example of an argument using modus ponens:

If an angle satisfies 90° < < 180°, then is an obtuse angle.

= 120°.

is an obtuse angle.

Modus tollens

.

(First premise is a conditional statement)

.

(Second premise is the negation of the consequent)

.

(Conclusion deduced is the negation of the antecedent)

The following is an example of an argument using modus tollens:

If it is raining, then there are clouds in the sky.

There are no clouds in the sky.

Thus, it is not raining.

Law of syllogism

In proposition logic the syllogism

Therefore,.

The following is an example:

If the animal is a Yorkie, then it's a dog.

If the animal is a dog, then it's a mammal.

Therefore, if the animal is a Yorkie, then it's a mammal.

We deduced the final statement by combining the hypothesis of the first statement with the conclusion of the second statement.

We also allow that this could be a false statement.

This is an example of the transitive property in mathematics.

Another example is the transitive property of equality which can be stated in this form:

.

.

Therefore,.

Validity and soundness

Deductive arguments are evaluated in terms of their validity and soundness

An argument is “**valid**” if it is impossible for its premises to be true while its conclusion is false. In other words, the conclusion must be true if the premises are true. An argument can be “valid” even if one or more of its premises are false.

An argument is “**sound**” if it is *valid* and the premises are true.

It is possible to have a deductive argument that is logically *valid* but is not *sound*. Fallacious arguments often take that form.

The following is an example of an argument that is “valid”, but not “sound”:

Everyone who eats carrots is a quarterback.

John eats carrots.

Therefore, John is a quarterback.

The example's first premise is false – there are people who eat carrots who are not quarterbacks – but the conclusion would necessarily be true, if the premises were true.

In other words, it is impossible for the premises to be true and the conclusion false.

Therefore, the argument is “valid”, but not “sound”.

False generalizations – such as "Everyone who eats carrots is a quarterback" – are often used to make unsound arguments.

The fact that there are some people who eat carrots but are not quarterbacks proves the flaw of the argument.

In this example, the first statement uses categorical reasoning, saying that all carrot-eaters are definitely quarterbacks. This theory of deductive reasoning – also known as term logic – was developed by Aristotle, but was superseded by propositional (sentential) logic and predicate logic.

Deductive reasoning can be contrasted with inductive reasoning, in regards to validity and soundness. In cases of inductive reasoning, even though the premises are true and the argument is “valid”, it is possible for the conclusion to be false (determined to be false with a counterexample or other means).

History

Aristotle, a Greek philosopher, started documenting deductive reasoning in the 4th century BC.^{[2]}

See also

Analogical reasoning

Argument (logic)

Argumentation theory

Correspondence theory of truth

Decision making

Decision theory

Defeasible reasoning

Fallacy

Fault Tree Analysis

Geometry

Hypothetico-deductive method

Inference

Inquiry

Legal syllogism

Logic and rationality

Logical consequence

Logical reasoning

Mathematical logic

Natural deduction

Peirce's theory of deductive reasoning

Propositional calculus

Retroductive reasoning

Subjective logic

Theory of justification

## References

*Cognitive Psychology*. Belmont, CA: Wadsworth. p. 578. ISBN 978-0-495-50629-4.

*Human Reasoning: The Psychology of Deduction*(Reprint ed.). Psychology Press. p. 4. ISBN 9780863773136. Retrieved 2015-01-26. In one sense [...] one can see the psychology of deductive reasoning as being as old as the study of logic, which originated in the writings of Aristotle.