What is deductive reasoning?

Introduction

Deductive reasoning is a method for analyzing a situation using general, proven facts to validate a more specific conclusion. The usual format for deductive reasoning is the syllogism, which is a logical argument with three statements: the major premise, the minor premise, and the conclusion Both the major premise and the minor premise must be true for the conclusion to be true.

Deductive reasoning starts with a known fact, such as “all fish have gills,” to which is added a second statement of proven fact, such as “a barracuda is a fish.” The third statement uses the first two to prove that “a barracuda has gills.” Deductive reasoning requires that the first two statements be true. If one or both are false, then the third statement is also false. An example of this would be “all fish have gills; a dolphin is a fish; therefore a dolphin has gills.” The conclusion is false because the minor premise is false: a dolphin is a mammal, not a fish, and it does not have gills.

Deductive reasoning and its counterpart,
inductive reasoning, are part of the philosophy of logic. Logic is a way of presenting arguments and then demonstrating the proof of those arguments using deductive reasoning, inductive reasoning, or some other method. The philosophy of logic studies reasoning, probability, and arguments that lead to demonstrating the cause of an occurrence. In addition, logic includes the study of fallacies, validity, and paradoxes.

Applications

Deductive reasoning is used in numerous different areas of study. Geometric theorems use known characteristics about an object to determine what it is. Debaters and analytical writers use deductive reasoning to support their positions; research studies use it to either prove or disprove the theory that they are testing. Sciences such as medicine, biology, chemistry, and meteorology use deductive reasoning as well. In medicine, signs and symptoms are first determined, and from them a diagnosis is made. Biology uses the characteristics of a creature to determine its classification. In chemistry, a solution is tested, and the results are then used to determine the chemicals that make up the solution. A meteorologist looks at cloud types, wind-flow position and direction (the jet stream), and weather in the area to forecast the weather for the future.

At times, what appears to be a logical conclusion based on deductive reasoning turns out to be incorrect. This faulty reasoning can occur because of illegitimate or irrelevant points, lack of information, or biased evidence. Some faulty reasoning is based on emotions or fear. Such an error in reasoning can happen when a medical patient has some of the symptoms of a disease but not all of them, or when the person’s signs and symptoms are too general to make a correct diagnosis. If there is a strong need to make a diagnosis so that treatment can begin, a physician may be forced to evaluate the signs and symptoms that are available to make a diagnosis.

History

Deductive reasoning began in the Greek classical period, which dates from 600 to 300 BCE. It was developed and described by philosophers such as Aristotle, Thales, and Pythagoras, among others. Aristotle’s writings on logic were the beginning of the use of the syllogism for the evaluation of judgments. During this period, examples of deductive reasoning included predicting the likelihood of crop success or failure and determining the accuracy of statements made by politicians.

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