Deductive Reasoning

What is deductive reasoning?

Deductive reasoning is a type of argument in which a general premise leads to a specific conclusion. It is also known as the inductive argument.

The premise of a deductive argument is usually a general law, rule, or principle that is accepted as true. Since the conclusion follows from that true premise, it is considered valid. This is why it is said that in deductive arguments the truth of the conclusion is inferred necessarily either with absolute necessity.

For example,

  • Premise: All dogs bark. My pet is a dog.
  • Conclusion: Necessarilymy dog ​​barks.

Deductive reasoning is used to understand specific phenomena (my dog ​​barks) based on the laws or principles that define them (all dogs bark).

Unlike inductive arguments, the conclusions of deductive reasoning do not provide new information, but only confirm the premise.

Deductive reasoning is one of the main types of reasoning, along with inductive and abductive.

Characteristics of deductive reasoning

  • Your premise is true. In an argument or deductive reasoning, the premise always contains a principle, law, or rule that is accepted as true.
  • It does not provide new information. The conclusion of inductive reasoning does not provide new information, but rather reaffirms the truth contained in the premise, applied to a specific case.
  • Their conclusions are considered valid. As long as the premises are true and the reasoning process is correct, the conclusion of a deductive argument is considered valid.
  • The validity of the conclusion depends on the form. Since the conclusion does not provide information other than the premise, its validity does not come from the content, but from the form of the reasoning, that is, from the internal coherence between the premise and the conclusion.
  • It can generate fallacies. When an attempt is made to construct a deductive argument based on a dubious premise or by making a mistake in the reasoning process, the conclusion is considered a fallacy, that is, it lacks the truth.
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It may interest you:

  • Reasoning
  • Deductive method

Types of deductive reasoning

Normally, deductive arguments are made up of a premise (divided into two complementary parts), and a conclusion. According to the different ways of constructing the premises, different types of deductive reasoning are formed: the syllogism, the modus ponndo pons and the modus tollendo tollens.


The syllogism is the deductive form par excellence, and its classification is attributed to Aristotle. In the syllogism, the first part of the premise is considered major, and the second part is considered minor.

For example,

  • Major premise: All bacteria are single-celled organisms.
  • Minor premise: The Escherichia coli it is a bacterium.
  • Conclusion: The Escherichia coli it is a unicellular organism.

modus ponndo pons either modus ponens

Also called statement of the antecedent. In this type of deductive reasoning, the first part of the proposes a condition. The second confirms it.

For example,

  • Premise 1: If the fingerprints on the gun match the defendant’s, the defendant is guilty.
  • Premise 2: The fingerprints on the gun match those of the defendant.
  • Conclusion: The defendant is guilty.

Modus tollendo tollens either modus tollens

Also called negation of the consequent. In this case, the first part of the premise is also conditional, but the second rejects it. In other words, if P implies Q, but Q is not true, then P is not true either.

For example,

  • Premise 1: If there is an infection, the patient must have a fever.
  • Premise 2: There is no fever.
  • Conclusion: The patient does not have an infection.

It may interest you: Syllogism

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Deductive reasoning and inductive reasoning

Unlike deductive reasoning, inductive reasoning occurs when the observation of repeated phenomena in a sample of cases leads to a general conclusion that is probable, but not valid on its own.

In contrast, deductive reasoning starts from a general truth or principle that is accepted as true, to formulate a specific conclusion about a phenomenon.

For this reason, it used to be said in the past that the inductive method goes from the particular to the general, and the deductive method goes from the general to the particular.

In the following table, we can see the comparison of an example of a deductive argument with an inductive one.

deductive argument inductive argument

All birds have feathers.
The penguin has feathers.
Necessarilythe penguin is a bird.

The eagle is a bird and it has feathers.
The chicken is a bird and it has feathers.
The penguin is a bird and it has feathers.
Probablyall birds have feathers.

It may interest you:

  • inductive reasoning
  • abductive reasoning
  • Argument Examples
  • Paradigm
  • Examples of inductive and deductive arguments.