What is Deductive Reasoning?

Deductive reasoning is also called as logical reasoning. Deductive reasoning is a term that is used to a reasoning method where we get conclusions logically from two or more premises. Premises are the statements which are used to derive at conclusions and it uses logic psychologically to come to a conclusion. Human brain uses deductive reasoning using higher cortical centers. Deductive reasoning is opposite to that of Inductive reasoning.

Examples of Deductive reasoning of two premise

Example 1

  • Statement 1: All fast foods are unhealthy.
  • Statement 2: Joan eats fast foods

Conclusion: According to deductive reasoning Joan eats unhealthy foods. This conclusion is derived from the two premises (Statement 1 and statement 2)

Example 2

  • Statement 1: We used to go to beach every Sunday
  • Statement 2: we went to beach today

Conclusion: By logical reasoning we can conclude that today is a Sunday.

Example 3

  • Statement 1: If the weather is good we would go to a tea party.
  • Statement 2: We went to the tea party

Conclusion: By deductive reasoning, we may conclude that the weather is good.

Deductive reasoning combining more than two premises:

Example 4

  • Statement 1: If the students are sincere and have time, they would finish the homework
  • Statement 2: Students finished the homework
  • Statement 3: Students have time

Conclusion: From the above three statements we can conclude that the students are sincere.

Example 5

  • Statement 1: My son would get a good salary if he is intelligent and hardworking
  • Statement 2: My son is getting a good salary
  • Statement 3: My son is hard working

Conclusion: – By logical or deductive reasoning we can conclude that her son is intelligent. So we are combining all the three states to come to a logical conclusion.

By seeing these examples we can clearly describe deductive reasoning as a logical reasoning which is related to the premises. It depends upon two or more premises. So to give a reason these premises or statements are needed.

Deductive reasoning is usually confused with Inductive reasoning. Deductively is logical. It needs two or more premises to support its reason. Whereas Inductive reasoning doesn’t need a logical thinking to arrive at a conclusion. It is derived just from a hypothesis or a statement.

For example All directors I have known are intelligent. From this, we conclude that all directors are intelligent. So we don’t use any logical sense to combine any two statements to come to a conclusion. It is just inferred from the statement.

Forms of deductive reasoning:

1. Law of Detachment: – This is a basic form of deductive reasoning. In this form of deductive, we are coming to a conclusion by a statement and a hypothesis. So we need a statement and a hypothesis to arrive at a reason.

For example:

Statement 1: If A is greater than B then A should be smart

Statement 2: A is smart

Conclusion: A is greater than B

2. Law of syllogism: – In this form of deductive we come to conclusion by combining the assumption of one statement with the conclusion of the other.

For example:

Statement 1: If I am not completing my thesis I will not get by approval

Statement 2: If I am not getting my approval I cannot write by exams

Conclusion: If I am not completing my thesis I cannot write my exams.

3. Law of Contrapositive: According to this form of Deductive reasoning, If the conclusion is false then the assumption is also a false statement.

Example:

Statement 1: – If he is shouting loudly then he is angry

Statement 2: – He is not shouting

Conclusion: – He is not angry. In this example, the conclusion is a negative statement since the hypothesis is a false one.

Another example for contrapositive law is

  •  Statement 1: If I know French I would see French movies
  •  Statement 2: I don’t know French.
  •  Conclusion: I cannot see French movies.

Here the conclusion is opposite that of the first hypothesis.

Psychological theories of deductive reasoning:

Numerous studies have been done about deductive reasoning in the fields of psychology, philosophy, and cognition. There are many controversies regarding the nature and logical connection to deductive reasoning. Though there are many theories of reasoning, the two most popular theories of deductive reasoning are.

  1. Mental logic theory
  2. Mental model theory.

The three things to be taken into account regarding Deductive reasoning will be the

  1. Relationship b/w the deductive reasoning and logic
    • Deductive reasoning is a psychological process whereas logic is not a psychological process. But yet it is accepted that there is a relationship b/w logic and deductive reasoning.
  2. Psychological models 
    • The mental logic theory and mental model theory describes the psychological model.
  3. The epistemology of deductive reasoning.

Evaluation of Deductive reasoning:

Deductive reasoning are evaluated on two scales namely

  • Validity and soundness.
    • A reasoning is said to be valid even if the premises and the conclusions are false whereas a reasoning is said to be sound only if the statements and the conclusion are true. There are many instances where the reasoning may be sound but not valid or vice versa.
    • In inductive reasoning even though the statement is true and the reasoning is valid but the conclusion may be false.
    • Deductive reasoning arguments may be valid or invalid. It depends upon the premises or hypothesis.

Scientist uses deductive reasoning in their experiments to come to a conclusion of their studies. There are many studies in deductive reasoning questioning the logical concerns behind the deductive reasoning. However logic comes into play is widely accepted by many scientist and psychologist.

Conclusion

So in concluding about deductive reasoning, it has to depend upon the premises to give a reason and supports the conclusion of the argument.