What is Deduction? Deduction or Deductive Reasoning

What is Deduction? Deduction or Deductive Reasoning

July 2, 2021 Off By Felso

Deduction is a way of reaching a judgment about individual events based on general principles. In short, we can say that we call the way of thinking that extends from the general to the specific and from the general to the general deduction. The reasoning that deduces the particular from the general is deductive.

Deductive reasoning does not give us any new information, it merely clarifies the premises and informs us about the particular.

It is called deduction because it starts from a general law or judgment. It consists of at least two premises, one of which is law or judgment, and a conclusion that must be deduced from these premises. The more correct and strong the law on which it is based, the more valid reasoning it is.

From a few premises given in deduction, a conclusion is drawn. This conclusion is present in the premises. The conclusion neither forgives nor adds anything new to the premises. The task of deduction is to reveal the conclusion implicit in the premises. In other words, we can look at deduction not as a method of increasing our knowledge, but as a method of analyzing the information at hand.

It is a method of reasoning that deduces the particular from the general and the particular from the general. Deduction derives new propositions that necessarily follow from propositions that are true or presumed to be true. If the premises are true, the conclusion is true by logical necessity.

For example:

Man is mortal
Ahmed is human.
So Ahmet is also mortal.

syllogism is a syllogism that comes from the whole. If it is true that all humans are mortal, then Ahmet must be mortal as well, since Ahmet is also human, it cannot be otherwise.


Francis Bacon, the founder of experimental science and the inductive method of knowledge, rejected deduction because it did not resort to experimentation, but because it was a purely intellectual reasoning. On the contrary, Hegel argued that only deduction is real and that induction cannot be achieved from the individual. According to him, the only valid method for idealism is the deductive method.

Deductive and inductive methods are the invention of Aristotle, who sees a close relationship between the universal and the particular (general and particular), and searches for ways to present this relationship in the most correct way. The deductive method from the general to the particular and the inductive method from the particular to the general have developed considerably since the 17th century. In particular, the dependence between these two methods, the use of both, has been realized in dialectical logic.

For example:

All humans are mortal.
Socrates is a human being.
So Socrates is mortal.

To give another example;

All metals expand when heated.
Copper is a metal.
Therefore, copper expands when heated.

The most distinctive feature of deduction is that it claims that the premises confirm the conclusion. With the realization of this assertion, the inference becomes valid at the same time. There may also be examples of deductions that lack logical validity. Namely:

All Thracians have gray eyes.
Einstein had gray eyes.
So, Einstein is Thracian.

In deductive reasoning, although the premises are true, the conclusion does not seem to be true. Since the conclusion is not true, it is not logically valid. If being a Thracian shares many common features such as a, b, c…, having one of these features does not require having the others, and having a single feature does not include that person in that group.

In addition, the universal proposition that all Thracians have gray eyes is also wrong. Einstein may have randomly gray eyes, but this single feature is not enough for Einstein to be a Thracian. Aristotle included only the deductive reasoning method in classical logic and gave importance to deduction. According to Aristotle, the mind seeks the truth in this way.

For example:

All elephants can fly.
Mistafa is an elephant.
Then Mistafa can fly too.

To give another example;

All women are flowers.
All flowers smell good.
So, all women smell good.

In the first of the examples above, a general-to-specific reasoning was used, and in the second, a general-to-general reasoning was used.

Prepared by: Sociologist Ömer YILDIRIM
Source: Omer YILDIRIM’s Personal Lecture Notes. Atatürk University Sociology Department 3rd Grade “Classical Logic” and “Modern Logic” Lecture Notes (Ömer YILDIRIM)