Evaluating an Argument in Action

1. Determining if Argument is Deductive and Inductive

Recall the definitions of inductive and deductive, as they relate to arguments.

When determining whether an argument is inductive or deductive, ask this question: “Is the inferential claim a claim of logical certainty?” If the answer is yes, the argument is deductive. If the answer is no, the argument is inductive.

You cannot properly evaluate a sophisticated argument without first identifying whether it is deductive or inductive.

terms to know

Deductive

An argument in which the inferential claim is a claim of logical certainty

Inductive

An argument in which the inferential claim is a claim of less than logical certainty

2. Evaluating Inductive and Deductive Arguments

Once you have determined whether an argument is inductive or deductive, ask this question: “Assuming all premises are true, do they support the conclusion?” For deductive arguments, is it possible for all of the premises to be true, and the conclusion false? For inductive arguments, is it probable for all premises to be true and the conclusion false? You are essentially trying to determine whether the premises guarantee the conclusion, or not.

2a. Valid, Invalid, Strong, or Weak

For deductive arguments, we use the terms valid and invalid:

• Valid: A deductive argument in which the premise(s) logically guarantee their conclusion.
• Invalid: A deductive argument in which the premise(s) do not logically guarantee their conclusion.

• Strong: In inductive argument in which the premises render the conclusion probable.
• Weak: An inductive argument in which the premises do not render the conclusion probable.

Here are some examples:

The last step in evaluating an argument is to check the factual claims. Whether the premises or conclusion are true or not does not enter into the determination of validity (deductive arguments) or strength (inductive arguments). However, validity and strength are important because they indicate that the premises lead to the conclusion.

terms to know

Valid

A deductive argument in which the premise(s) logically guarantee their conclusion

Invalid

A deductive argument in which the premise(s) do not logically guarantee their conclusion

Strong

An inductive argument in which the premises render the conclusion probable

Weak

An inductive argument in which the premises do not render the conclusion probable

2b. Sound, Unsound, Cogent, or Uncogent

After that, we simply have to decide whether we accept the premises. If the argument has been determined to be valid or strong, we simply need to ask our question of fact: “Are all of the premises true?” This will determine whether a valid argument is sound or unsound (deductive arguments), or a strong argument is cogent or uncogent (inductive arguments).

Let's make these evaluations more precise.

• A sound argument is a deductively valid argument with all true premises, while an unsound argument is simply any deductive argument that is not sound.
• Similarly, a cogent argument is an inductively strong argument with all true premises, while an uncogent argument is any inductive argument that is not cogent.

terms to know

Sound

A deductively valid argument in which all premises are true

Unsound

A deductive argument that is not sound

Cogent

An inductively strong argument in which all premises are true

Uncogent

An inductive argument that is not cogent

3. Practice Evaluating an Argument

Let’s apply what you’ve learned to a few examples. In these examples evaluate the argument by determining if it is:

1. Inductive or deductive
2. Valid, invalid, weak, or strong
3. Sound, unsound, cogent, or uncogent

try it

Evaluate the following argument by first determining whether it is deductive or inductive, then evaluating its inferential claim, and, finally, evaluating its factual claims.

If you are in Texas, you are in the United States. You are in the United States. Therefore, you are in Texas.

This is a deductive argument. The conclusion follows from the meaning of an if-then statement, not from facts about geography. Note that the argument includes nothing related to causation. It is about definition. Next, evaluate the inferential claim: does the inference attain logical certainty, or can you imagine the premises being true, and the conclusion false? For most students, both premises are true and the conclusion is false (e.g., if you are in Michigan). This means that it is invalid because even if you happen to be in Texas, that is not entailed by the premises. Since it is invalid, it must be unsound because it is not a satisfactory deductive argument.

try it

Evaluate the following argument by first determining whether it is deductive or inductive, then evaluating its inferential claim, and, finally, evaluating its factual claims.

In the past, when I have had more than five drinks, I have become ill. Therefore, if I have ten drinks, I will become extremely ill.

This argument is inductive because it relies on cause and effect (it is a prediction because it is forecasting a similar cause and effect in the future). When we ask our inference question, we see that the conclusion is likely, given the premise. Therefore, the argument is strong. If the single premise is true, it is cogent; otherwise, it is uncogent.

try it

Evaluate the following argument by first determining whether it is deductive or inductive, then evaluating its inferential claim, and finally evaluating its factual claims.

All birds can fly. Polly is a bird. Therefore, Polly can fly.

This is a deductive argument because it proceeds from the definition of “all” rather than facts about biology. Next, evaluate the inferential claim. Can you imagine an instance in which the premises are true, but the conclusion is false? The answer is no, therefore, this argument is valid. Finally, evaluate the factual claims. While “Polly is a bird” might be true, the statement that “all birds can fly” is not. There are several species of flightless birds (penguins, ostriches, kiwis, etc.). Therefore, this argument is unsound.