• Understand different types of arguments
• Use logic to see if a statement is true
• Identify logical fallacies

Argument Types

A logical argument is a claim that a set of premises support a conclusion. There are two general types of arguments: inductive and deductive arguments.

argument types

An inductive argument uses a collection of specific examples as its premises and uses them to propose a general conclusion.

 

A deductive argument uses a collection of general statements as its premises and uses them to propose a specific situation as the conclusion.

Many scientific theories, such as the big bang theory, can never be proven. Instead, they are inductive arguments supported by a wide variety of evidence. Usually, in science, an idea is considered a hypothesis until it has been well tested, at which point it graduates to being considered a theory. The commonly known scientific theories, like Newton’s theory of gravity, have all stood up to years of testing and evidence, though sometimes they need to be adjusted based on new evidence. For gravity, this happened when Einstein proposed the theory of general relativity.

Evaluating Arguments

Inductive arguments can never be truly valid or invalid. Inductive arguments are evaluated by how strong or weak the evidence of the argument suggests it is true. An inductive argument is considered strong when the assumption that the premises are true is made and it is improbable for the conclusion to be false. An inductive argument is considered weak when the assumption that the premises are true is made and the conclusion is not likely to occur given the premises of the argument.

evaluating inductive arguments

An inductive argument is never able to prove the conclusion true, but it can provide either weak or strong evidence to suggest it may be true.

 

Strong inductive arguments are characterized by a high probability of the conclusion being true given true premises.

 

Weak inductive arguments have a low probability of the conclusion being true given true premises.

Take a moment to evaluate the following two inductive arguments. Do you think they are a strong or weak argument?

1. When I went to the store last week I forgot my purse, and when I went today I forgot my purse. I always forget my purse when I go to the store.
2. Every day for the past year, a plane flies over my house at [latex]2[/latex]PM. A plane will fly over my house every day at [latex]2[/latex]PM.

Show Solution

1. The premises are:

   I forgot my purse last week.
   I forgot my purse today.

   The conclusion is:

   I always forget my purse.

   Notice that the premises are specific situations, while the conclusion is a general statement. This is a fairly weak argument since it is based on only two instances.

2. This is a stronger argument since it is based on a larger set of evidence.

A deductive argument is more clearly valid or not, which makes them easier to evaluate. Validity is not about evaluating the truth of an argument’s premises, but evaluating the form of the argument. It strictly relates the logic between the given premises and conclusions. It is possible to have valid arguments that have premises that are false statements and invalid arguments with premises that are true statements.

We can go one step further in talking about the validity if we also talk about the soundness of an argument. A sound argument is a valid argument with all true premises. Otherwise, a deductive argument is unsound.

Evaluating deductive arguments

A deductive argument is considered valid assuming all the premises are true, and the conclusion follows logically from those premises. In other words, the premises are true, and the conclusion follows necessarily from those premises.

 

A deductive argument is considered sound if the argument is valid and all the premises are factually true statements.

Determine if the following argument is valid or invalid

• Premise 1: All primates are mammals.
• Premise 2: All chimpanzees are mammals.
• Conclusion: All chimpanzees are primates.

Show Solution

This argument is invalid. Although the premises and conclusion of this argument are true statements by themselves, the strength of the logical reasoning between the premises and the conclusion is not valid.