What Is Deductive Reasoning?

In general, reasoning involves evaluating information and generating logical arguments. To do this, physical, pictorial or linguistic symbols have to be manipulated cognitively. Two main forms of reasoning may be identified: inductive reasoning and deductive reasoning.

Deductive reasoning refers to reasoning in which one moves cognitively from the general to the specific. In other words, one begins with the general idea or rule and then deduces instances of this. Syllogisms use deductive reasoning, and typically comprise three components:

• a general premise is made;
• a statement of fact is presented that relates to the first premise; and
• a statement or assertion is made that represents a conclusion deduced from the prior premise and fact.

The issue in a syllogism is whether the final statement is true or not. For example, consider the following syllogism, which is attributed to Woody Allen, the American comedian:

Premise:          Aristotle is a man. A.

Fact:                I am a man. B.

Conclusion:     Therefore, I am Aristotle! C.

Given the premise A, and the fact B, can we deduce that C is correct? No. Human beings other than Aristotle fall into the category ‘man’. The fact that B is true does not enable us to logically deduce that the person referred to in C ‘is Aristotle’. This is an example of an invalid syllogism. In a valid syllogism, the conclusion follows logically from the premise, for example:

Premise:          Boats and only boats float on water. A.

Fact:                X is floating on water. B.

Conclusion:     Therefore, X is a boat. C.

Research has shown that people perform better on valid rather than invalid syllogisms.