Summary

Arguments come in two forms: deductive and inductive. A deductive argument is intended to provide logically conclusive support for a conclusion; an inductive one, probable support for a conclusion. Deductive arguments can be valid or invalid; inductive arguments, strong or weak. A valid argument with true premises is said to be sound. A strong argument with true premises is said to be cogent.

Evaluating an argument is the most important skill of critical thinking. It involves finding the conclusion and premises, checking to see if the argument is deductive or inductive, determining its validity or strength, and discovering if the premises are true or false. Sometimes you also have to ferret out implicit, or unstated premises.

Arguments can come in certain patterns, or forms. Two valid forms that you will often run into are modus ponens (affirming the antecedent) and modus tollens (denying the consequent). Two common invalid forms are denying the antecedent and affirming the consequent.

Analyzing the structure of arguments is easier if you diagram them. Argument diagrams can help you visualize the function of premises and conclusions and the relationships among complex arguments with several sub-arguments.

Assessing very long arguments can be challenging because they may contain lots of verbiage but few or no arguments, and many premises can be implicit. Evaluating long arguments, though, requires the same basic steps as assessing short ones: (1) ensure you understand the argument, (2) locate the conclusion, (3) find the premises, and (4) diagram it to clarify logical relationships.

Common Argument Forms

Valid Argument Forms

-Affirming the Antecedent (Modus Ponens)
If p, then q.
p.
Therefore, q.
Example: If Spot barks, a burglar is in the house.
Spot is barking.
Therefore, a burglar is in the house.

-Denying the Consequent (Modus Tollens)
If p, then q. Ex. If it's raining, the park is closed.
Not q. The park is not closed.
Therefore, not p. Therefore, it's not raining.
Example: If it's raining, the park is closed.
The park is not closed.
Therefore, it's not raining.

-Hypothetical Syllogism
If p, then q.
If q, then r.
Therefore, if p, then r.
Example: If Ajax steals the money, he will go to jail.
If Ajax goes to jail, his family will suffer.
Therefore, if Ajax steals the money, his family will suffer.

-Disjunctive Syllogism
Either p or q.
Not p.
Therefore, q.
Example: Either we light the fire or we will freeze.
We will not light the fire.
Therefore, we will freeze.

Invalid Argument Forms

-Affirming the Consequent
If p, then q.
q.
Therefore, p.
Example: If the cat is on the mat, she is asleep.
She is asleep.
Therefore, she is on the mat.

-Denying the Antecedent
If p, then q.
Not p.
Therefore, not q.
Example: If the cat is on the mat, she is asleep.
She is not on the mat.
Therefore, she is not asleep.

	Created by Erica Lawrence over 9 years ago

Critical Thinking