Deduction

When you argue using deduction, you reach a conclusion by starting with a general principle or universal truth (a major premise) and applying it to a specific case (a minor premise). Deductive reasoning is often structured as a syllogism, a logical structure that uses the major premise and minor premise to reach a necessary conclusion. Let’s use the same example about exercise that we used to demonstrate induction, but now we’ll develop a syllogism to argue deductively:

major premise: Exercise contributes to better health.
minor premise: Yoga is a type of exercise.
conclusion: Yoga contributes to better health.

The strength of deductive logic is that if the first two premises are true, then the conclusion is logically valid. Keep in mind, though, that if either premise is false (or questionable in any way), then the conclusion is subject to challenge. Consider the following:

major premise: Celebrities are role models for young people.
minor premise: Lindsey Lohan is a celebrity.
conclusion: Lindsey Lohan is a role model for young people.

As you can see in this example, the conclusion is logically valid—but is it true? You can challenge the conclusion by challenging the veracity of the major premise—that is, whether all celebrities are role models for young people.

Deduction is a good way to combat stereotypes that are based on faulty premises. Consider this one:

major premise: Women are poor drivers.
minor premise: Ellen is a woman.
conclusion: Ellen is a poor driver.

Breaking this stereotype down into a syllogism clearly shows the faulty logic. Perhaps some women, just as some men, are poor drivers, but to say that women in general drive poorly is to stereotype by making a hasty generalization. Breaking an idea down into component parts like this helps expose the basic thinking, which then can yield a more nuanced argument. This example might be qualified, for instance, by saying that some women are poor drivers; thus, Ellen might be a poor driver.