The reasoning It is the mental process and the consequences of reasoning (the activity that consists of organizing and structuring ideas to reach a conclusion). Deductive, for its part, is what comes from the deduction (the logical method that leads from the universal to the particular).

It is known as deductive reasoning, therefore, to the activity of the mind which enables necessarily infer a conclusion from a series of premises. This means that, starting from the general, one arrives at the particular.

Deduction

Deductive reasoning starts from different premises to necessarily infer a conclusion.

Complementary concepts

To understand the concept of deductive reasoning, we must bear in mind others that complement it, such as the following:

* argument: it is a reason or proof that allows to justify or refute something, to affirm that it is true or false. In other words, it is a speech that it has a very clear objective, and allows expressing a reasoning orally or in writing;

* proposition: both in logic and in philosophy, it is each of the entities that carry the values of truth (that is, they indicate to what degree a statement is true; for bivalent classical logic, one can only speak of “true” or “false”);

* premise: logic defines this concept as any proposition found before the conclusion. It should be noted that if the argument is valid, then the set of premises implies the conclusion, although this does not make a proposition a premise or not, but rather its position in the argument what counts;

* conclusion: from the point of view of logic, it is a proposition that is in the last part of an argument, after the premises. In the same way as the premise, for a proposition receives the role of conclusion it does not matter if the argument is valid, it is enough that it is in last place;

Investigation

Despite the validity of the form, deductive reasoning can lead to a false conclusion.

* axiom: it is a proposition that is taken as evident, for which a prior demonstration is not required;

* rules of inference: also known as transformation rules, are logical forms or functions that take premises to analyze their syntax and draw one or more conclusions.

Characteristics of deductive reasoning

Taking all the above into account, we can observe the formal definition of deductive reasoning: it is a well-defined sequence of formulas, among which the last one is designated as the conclusion of the entire argument and the rest can be axioms or premises, or also direct inferences from rules of inference.

A example deductive reasoning is as follows: “All dogs have four legs / Bobby is a dog / Bobby has four legs”. As can be seen, the conclusion (“Bobby has four legs.”) derives directly from the original premise, which is universal (“All dogs have four legs”).

It is often said that deductive reasoning begins with a major premise and is complemented with a minor premise to arrive at the conclusion:

Major premise: “All human beings, at some point, will die”.
Minor premise: “Bruno is a human being”.
Conclusion: “Bruno, at some point, will die”.

Valid form, false conclusion

It is important to note that deductive reasoning may be valid in its form, but lead to a false conclusion when starting from a premise that is not true: “Women are always blonde / Oprah Winfrey is a woman / Oprah Winfrey is blonde”. In this case, the deduction is logic, but the original premise is false, leading to a false conclusion as well.

As can be seen in all the examples, deductive reasoning does not always lead us to a true conclusion; in the same way, it does not always offer us information detailed or precise, despite starting from the general to arrive at the particular.