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 an inductive argument.

The premise of a deductive argument is usually a general law, rule, or principle that is accepted as true. Since the conclusion is inferred 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 or with absolute necessity.

For instance,

  • Premise: All dogs bark. My pet is a dog.
  • Conclution: Necessarily, my 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

  • Its 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 doubtful premise or by erring in the reasoning process, the conclusion is considered a fallacy, that is, it is untrue.

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 putting ponies and the modus tollendo tollens.

Syllogism

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 is considered minor.

For instance,

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

Modus putting ponies or modus ponens

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

For instance,

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

Modus tollendo tollens or 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 instance,

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

It may interest you: Syllogism

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 probable general conclusion, but not valid by itself.

Instead, 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 another inductive one.

Deductive argumentInductive argument

All birds have feathers.
The penguin has feathers.
Necessarily, the penguin is a bird.

The eagle is a bird and has feathers.
The hen is a bird and has feathers.
The penguin is a bird and has feathers.
Probably, all birds have feathers.

It may interest you:

  • Inductive reasoning
  • Abductive reasoning
  • Examples of arguments

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