Inductive and Deductive Reasoning

The philosophical method hinges on one’s ability to make sound, reasoned arguments. After all, ideas about how mankind should live, behave and conduct business aren’t worth much if they can’t stand up to logical scrutiny. Ancient philosophers established two main types of reasoning to test the validity of their observations and construct rational arguments: inductive and deductive reasoning. Though flawed, both inductive and deductive reasoning provide the basic framework for the kind of logical analysis that drives scientific research and discovery.

Inductive Reasoning

Inductive reasoning is a process by which a person makes a generalization based on specific, individual experiences. The process begins with observation. A person collects data through the five senses, then analyzes the gathered information to draw a general conclusion that may be applied to other situations. It is the most basic form of reasoning in existence.

Of course, conclusions drawn from inductive reasoning may not necessarily be true, even if a person’s observations, or premises, are accurate. Inductive reasoning is useful mainly for developing hypotheses that can later be tested more thoroughly, rather than arriving at laws or facts. As such, inductive reasoning is highly vulnerable to bias and logical fallacies.

For example, a person may observe that every time the governor of his state passes a business regulation, a business in his neighborhood folds. Based on that observation, the person might conclude that regulations only cause businesses to fail. However, that conclusion contradicts reality: thousands of businesses thrive around the world in spite of numerous strict regulations, and some because of them. The fact that businesses closed in the observer’s neighborhood after the governor passed new regulations may have simply been a coincidence resulting from other causes. In any case, more information is needed to determine whether regulations are really detrimental to the observer’s local economy.

Despite the inherent unreliability of inductive reasoning, it is one of the most essential problem-solving tools humans possess. Whether they realize it or not, people use inductive reasoning nearly every day. A person seeing dark clouds on the horizon in the morning will assume the weather will turn rainy in the afternoon and will take an umbrella to work. A person getting burnt, inedible bread from his toaster several times in a row will assume the toaster is malfunctioning and seek to repair it. A mother seeing her child break out in hives after eating peanut butter will conclude that her child has an allergy to peanuts. Though all of these conclusions may be false, they are all still logical and motivate observers to take action to improve their situations.

Some widely accepted scientific theories have also resulted from induction, like gravity and evolution. Standardized tests evaluate how well students can reason inductively by asking them to complete visual patterns and numbered sequences.

There are several types of inductive reasoning:

Generalization: a population is assumed to have the same characteristics observed in a sample.
Simple Induction: premises made about a sample are applied to an individual case.
Analogy: two things that share a characteristic may have other characteristics in common.
Causal Inference: an effect appearing during or after an event may have been caused by the event.
Statistical Syllogism: a generalization about a sample is applied to an individual case.
Prediction: events observed in the past may occur in the future under similar conditions.

An inductive argument can be described as either weak or strong. Weak induction occurs when the connection between the premise and the conclusion is highly tenuous. An example of this would be the following statement: “I know two welfare recipients, and they both use drugs. Therefore, most welfare recipients probably use drugs.” In this case, the sample size is far too small to draw such a broad conclusion.

With strong induction, however, the premise and conclusion are closely linked. An example of strong induction would read something like this: “So far, my professor has given a failing grade for every essay submitted late. If I am late in submitting my essay, it will probably receive a failing grade.” The argument is strong because both the premise and conclusion only concern the observed behavior of a specific individual.

Deductive Reasoning

Deductive reasoning is essentially the opposite of inductive. Deductive reasoning begins with a generalization as part of its premise to draw a conclusion about a specific, individual instance. The generalization applied is usually a stated law or theory—something that has not yet been proven untrue. Deductive reasoning is mostly an exercise in proper argument construction, so instead of being simply classified as either “false” or “true,” deductive arguments are labeled as being either valid or invalid, sound or unsound.

A valid argument is not necessarily a true one; it’s one that’s simply logical in its construction. Logically, a false conclusion must result from false premises, and a true conclusion must result from true premises. For example, the following is a valid argument:

Ice cream contains saturated fat.
Everyone who consumes saturated fat develops heart disease.
Therefore, everyone who eats ice cream will develop heart disease.

Because the argument’s second premise is false, the argument is unsound. But because a false premise led to a false conclusion, the argument is valid. A true conclusion cannot result from a false premise. An argument is only considered to be sound when it is valid and its premises are true. A true conclusion resulting from false premises would indicate an invalid (i.e., badly constructed) argument.

There are two types of deductive reasoning:

Law of Detachment: a conclusion is drawn from a hypothesis about a generalization (A of B = C).
Law of Syllogism: draws a conclusion from two conditional statements by combining the hypothesis of one with the conclusion of the other (AB + CD = AD).

Because deduction relies more on proven theories, it is considered a higher and more reliable form of reasoning than induction. However, being able to reason deductively requires having the right facts on hand at any given time.

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{ 1 comment… read it below or add one }

Galbe July 28, 2012 at 7:29 am

“Logically, a false conclusion must result from false premises, and a true conclusion must result from true premises.”
This simple sentence above is the whole spirit of what we call “proof by contradiction” and “reductio ad absurdum” (reduction to absurdity) in logic.
Neat article.

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