Phil XXI May
Two requirements for an argument to be good (rationally be strong for you):
• Its conclusion must follow from its premises
• Its premises must be reasonable for you to accept
- An argument whose premises, if true, would support its conclusion, is “well
- An argument is well-formed only if it is valid or cogent
- An argument is valid if its premises are true, its conclusion must be true
Ie. My van is parked where I left it or my van has been stolen. My van is
not parked where I left it. My van has been stolen. (validity)
- An argument is cogent if its premises are true, they provide reason to think
the conclusion is true, but they could be true yet the conclusion false
Ie. Most students in this room are under 25. The person in the top left
seat is a student in this room. That person is under 25.
Whether or not an argument is valid is simply a matter of its form!
How do we display the form of an argument?:
• In some cases, we can use sentential logic
Ie. 1. My van is parked where I left it.
2. My van is not parked where I left it.
3. My van has been stolen.
P = My van is parked where I left it
Q = My van has been stolen
1. P or Q
2. ~ P
(This is a valid argument pattern. Any argument with this same form would be
valid. If its premises were true, its conclusion would also be true. This pattern is
“argument by elimination.” The variables must stand for declarative sentences.)
3. P and Q
1. P and Q
1. If P, then Q
3. Q (“If, then” sentence is called a Conditional. If [antecedent], then [consequent]. In
a true conditional, if the antecedent is true, so is the consequent. The truth of the
antecedent is sufficient for the truth of the consequent. In a true conditional, the