Dr Mc’s Philosophy 100

Some Valid Argument Patterns

Although 100 is not a logic class, you still need to know something about arguments! Below are some common valid argument patterns, and below those are some common patterns that people sometimes mistake for valid patterns. There is a fun interactive site at: http://philosophy.hku.hk/think/arg/valid2.php. I recommend it.

Remember the distinguishing feature of valid arguments—if their premises are true, then their conclusion is true… no exceptions. Keep in mind that lots of good arguments are inductive. I am not providing inductive patterns here, as cogency is not, strictly speaking, a matter of pattern. Also, remember that just being valid (or cogent) isn’t enough to make an argument a good one.

Some valid argument patterns:

(the letters stand for declarative sentences)

1. If P, then Q

2. P

3. Q

(modus ponens or affirming the antecedent)

1. If P, then Q

2. Not Q

3. Not P

(modus tollens or denying the consequent)

1. P

2. Q

3. P and Q

(conjunction)

1. P or Q

2. Not P

3. Q

(disjunctive syllogism or argument by elimination)

1. P

2. P or Q

(addition—to understand this one, remember that a disjunction is true if at least one of its disjuncts is)

1. If P, then Q

2. If Q, then R

3. If P, then R

(hypothetical syllogism or chain argument)

EVIL LOOK-ALIKES

1. If P, then Q

2. Q

3. P

INVALID

1. If P, then Q

2. Not P

3. Not Q

INVALID

1. P or Q

2. P

3. Not Q

INVALID (to understand why, remember that a disjunction is true if at least one of its disjuncts is… so it’s true if they both are)

Posted 2006/09/12