We explain what the deductive method is and the ways in which it can be used. Also, examples and what the inductive method is.
What is the deductive method?
The deductive method or reasoning is a argument in which the conclusion is necessarily inferred from the premises. For example:
- The human being has the capacity to reason.
- Pedro is a human being.
- Pedro has the ability to reason.
For formal logic, the deduction is made up of a sequence that contains a certain number of premises and a conclusion. From the truth of the premises the truth of the conclusion is derived and guaranteed.
The deductive method goes from the general to the particular. The inductive method, on the contrary, goes from the particular to the general. Deductive reasoning is “top-down” and inductive is bottom-up.
See also: Scientific method
Characteristics of the deductive method
Some characteristics of the deductive method are:
- It goes from the general to the particular.
- Use top-down reasoning.
- Its conclusion is contained in advance in its own premises.
- If the premises are true, the conclusion will be true.
Uses of the deductive method
The deductive method can be used in two ways:
- Direct. It starts from a single premise that is not contrasted with others around it. This premise is considered an axiom. An axiom is the starting point of a scientific theory whose truth is shared by the entire academic community. An example of this is the law of gravity: that all objects fall to the ground is an indisputable axiom.
- Hint. It starts from a pair of premises: the first contains a universal statement and the second a particular one. The conclusion is obtained from the contrast between the two. This is what is considered traditional logical reasoning or syllogism and is the way to guarantee the permanence of the validity of the reasoning.
The validity of the premises determines the validity of the conclusion. This does not determine the truth or falsity of the argument. It is possible to start from false premises and deduce true or false conclusions, without making the logic of the reasoning invalid.
On the other hand, deductive reasoning gives rise to two more complex methods:
- axiomatic-deductive method. A set of theorems (propositions) is extracted from a set of axioms (premises) given in advance, using a series of logical reasoning.
- Hypothetico-deductive method. Based on the observation of a phenomenon, an interpretive hypothesis is stated which is then subjected to comparison by logical deductive reasoning. This is the method that scientific knowledge uses.
Examples of the deductive method
Many syllogisms use the deductive method. For example:
- Premise 1. All dogs are mortal.
Premise 2. Pluto is a dog.
Conclusion. Pluto is deadly. - Premise 1. Cows don't fly.
Premise 2. Animals that fly have wings.
Conclusion. Cows don't have wings. - Premise 1. Venezuelans are Caribbean.
Premise 2. María is Venezuelan.
Conclusion: María is Caribbean. - Premise 1. The planets are round.
Premise 2. The Earth is a planet.
Conclusion. The Earth is round. - Premise 1. The murderer was a man.
Premise 2. Martha is a woman.
Conclusion. Marta is not the murderer.
In all these deductive reasoning the validity of the argument is maintained beyond the truth or falsity of the premises. This is because deductive reasoning points to the formal aspect of the argument and not its content. If the content of each proposition is replaced by a generalization, the formal aspect of the argument is obtained.
- Premise 1. All X is Y
Premise 2. A is X
Conclusion. A is Y
What is the inductive method?
The inductive method is a reasoning whose procedure is opposed to deductive. Inductions go from the most particular to the most general.
Induction uses observation, recording and contrasting information to build general premises that can serve as support or explanation for the proposed generalization. It is often said that in an induction there is a leap of faith with respect to what is stated, since general conclusions are induced from a particular observation, which sometimes leads to an incorrect conclusion. For example:
- Premise 1. The washing machine broke.
Premise 2. The toaster broke.
Conclusion. All appliances break.
We can think of an example of induction where there is not a movement of truth transfer, but rather an inductive deduction:
- Premise 1. My father died.
Premise 2. My father was a man.
Conclusion. Men die.
References
- Gamut, LTF, & Durán, C. (2002). Introduction to logic. Buenos Aires, Argentina: Eudeba.
