Deductive reasoning starts from different premises to necessarily infer a conclusion.
Reasoning is the mental process and consequences of reasoning (the activity that consists of organizing and structuring ideas to reach a conclusion). Deductive , on the other hand, is what comes from deduction (the logical method that leads from the universal to the particular).
Known as deductive reasoning , therefore, is the activity of the mind that allows a conclusion to be necessarily inferred from a series of premises . This means that, starting from the general, we reach the particular.
Complementary concepts
To understand the concept of deductive reasoning, we must keep in mind others that complement it, such as the following:
* argument : this is a reason or proof that allows the justification or refutation of something, to affirm that it is true or false. In other words, it is a speech that has a very clear objective, and allows reasoning to be expressed orally or in writing;
* proposition : both in logic and philosophy, it is each of the entities that carry the truth values (that is, they indicate to what degree a statement is true; for classical bivalent logic, one can only speak of "true" or "fake");
* premise : logic defines this concept as any proposition that is 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 it is its position in the argument that counts;
* conclusion : from the point of view of logic, it is a proposition that is found in the last part of an argument, after the premises. In the same way as the premise, for a proposition to receive the role of conclusion it does not matter if the argument is valid, but it is enough that it is in last place;
Despite the validity of the form, deductive reasoning can lead to a false conclusion.
* axiom : this is a proposition that is taken as evident , for which a prior demonstration is not required;
* inference rules : also known as transformation rules , they are logical forms or functions that take premises to analyze their syntax and yield one or more conclusions.
Characteristics of deductive reasoning
Taking all of 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 that start from inference rules .
An example of deductive reasoning is the following: “All dogs have four legs / Bobby is a dog / Bobby has four legs.” As you can see, 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 by a minor premise to reach 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 keep in mind that deductive reasoning can 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 logical , but the original premise is false, leading to a false conclusion.
As can be seen in all the examples, deductive reasoning does not always lead us to a true conclusion; Likewise, it does not always offer us detailed or precise information , despite starting from the general to reach the particular.