In Short

Deductive reasoning takes two premises, which may be rules, laws, principles, or generalizations, and forms a conclusion based upon them. In order to be valid, a deductive argument must have premises that are true and a conclusion that logically follows from those premises, without trying to go beyond them. When you understand how these arguments work, you will know how to construct your own strong arguments. You will also avoid being influenced or persuaded by faulty deductive reasoning when you recognize it and see its flaws.

Skill Building Until Next Time

■ Find a deductive argument in print. Put it in the form of a diagram, listing the major premise, minor premise, and conclusion. Is it valid? If not, why?

■ The next time you need to persuade someone to do something, such as eat at your favorite restaurant instead of theirs or see the movie you prefer, argue for your choice using deductive reasoning.

Misusing Deductive Reasoning— Logical Fallacies


In this lesson you will see how the relationship between deductive reasoning and logic works, or does not work. This lesson explores four of the most common logical fallacies that make deductive reasoning fall apart.

Lesson 12 explored the characteristics of a valid deductive argument. You know that you need two premises which are true, and a conclusion that logically follows from them without assuming or inferring any information not contained in the premises. An invalid argument contains one or more errors. It might have a factual error, such as a premise that is not true, or a conclusion that is not supported by the premises. Or, it may contain an error in logic. This type of error is known as a fallacy.

There are a number of logical fallacies that can occur in deductive arguments. There are four major logical fallacies:

1. Slippery Slope

2. False Dilemma

3. Circular Reasoning

4. Equivocation

Each of these will be explained in detail in the next sections.

The argument might have two true premises, and a conclusion that takes them to an extreme. This is known as the slippery slope fallacy. Or, it might be a false dilemma fallacy, which presents in its major premise just two options ("either-or") when in reality there are others. In circular reasoning, also known as begging the question, there is just one premise, and the conclusion simply restates it in a slightly different form. And finally, equivocation uses a word twice, each time implying a different meaning of that word, or uses one word that could mean at least two different things.

Arguments intended to convince or persuade may be believable to many listeners despite containing such fallacies, but they are still invalid. Recognizing these fallacies is sometimes difficult. But it is important to be able to do so to prevent being mislead, or persuaded by faulty logic.

Friendly Persuasion

Friendly Persuasion

To do this successfully you need to build a clear path of action by using tools if necessary. These tools would be facts, evidence and stories which you know they can relate to. Plus you always want to have their best interests at heart, in other words, you know what is good for them

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