Reasoning is the process of making inferences from a body of information. Given the information that Zeno is a spider, it is reasonable to conclude that Zeno has eight legs. Likewise, given the information that Leda’s room is 10 ft wide and 15 ft long, it is reasonable to conclude that the area of the room is 150 sq ft.

Suppose you are told that all gleeps are dovines, all dovines have green eyes, and Blimpsey is a gleep. You are now asked “What is the colour of Blimpsey’s eyes?” Even though you have no idea what gleeps and dovines are, and who Blimpsey is, you will not have any difficulty in inferring that Blimpsey’s eyes are green. The process that leads to such conclusions is reasoning.

(1)	a.	Zeno is a spider.          reasoning Therefore, Zeno has eight legs.
	b.	Leda’s room is 10 ft wide and 15 ft long.          reasoning Therefore, the area of Leda’s room is 150 sq ft.
	c.	All gleeps are dovines. All dovines have green eyes. Blimpsey is a gleep.          reasoning Therefore, Blimpsey’s eyes are green.

The propositions (statements) on the basis of which a conclusion is made are called premises. For instance,

(2)	All gleeps are dovines.	premise 1
	All dovines have green eyes.	premise 2
	Blimpsey is a gleep.	premise 3
	Therefore, Blimpsey’s eyes are green.	conclusion

We may now say that reasoning is the process of arriving at a conclusion from a given set of premises:

Structure of reasoning

premises         reasoning conclusion

We use reasoning when we calculate the amount of money we need to pay the grocer, decide whether it is safe to invest money in a new venture, and check for inconsistencies in an electoral campaign. Reasoning can be used for finding answers to questions, justifying an answer, or critically evaluating the credibility of an answers given. If you are told that Leno is a spider, and asked how many legs Leno has, you use reasoning to arrive at the answer that Leno has eight legs (using reasoning to arrive at answers). And if you are told that Mohanan’s grandmother has eight legs, you use reasoning to judge this statement to be false (using reasoning in critical evaluation.)

Contributed by K P Mohanan ().

See also: rules of inference, logic and reasoning, types of reasoning, classical and non-classical logics, validity of reasoning.