PHI 1101: Lecture 3 Sept 27 2011
An argument intended to provide logically
A deductive argument is intended to provide conclusive support for its conclusion
Final, definitive, undeniable support
The structure of some arguments is deductive
When arguments structured this way are good, they guarantee their conclusion.
All philosophers are smart.
Macdonald is a philosopher.
So MacDonald must be smart.
I’m taller than Aimee.
Aimee is taller than Melissa.
So, I’m taller than Melissa.
If you drove through town, you drove right past my house.
And you did drive right through town.
So, you must have driven right past my house.
A valid deductive argument, the two premises guarantee the ultimate truth in the conclusion. The first two
premises depend on each to prove the conclusion
In each case, if the premises offered really are true, the conclusion must also be true, and we therefore
describe that argument as being valid.
Pigs have wings. Any animal with wings can fly.
So, pigs can fly.
The premises are both false.
The conclusion is also false.
Do the premises support the conclusion?
Yes the structure of the argument is valid. Accordingly, if an argument has a combination of false premises
and false conclusion, it is still valid.
New Brunswick is west of Ontario.
Every province west of Ontario is famous for harvesting lobster.
Thus, New Brunswick is famous for harvesting lobster.
This argument has a combination of false premises and a true conclusion. Is it valid? Yes.
If you’re reading this statement, you are alive.
You are reading this statement.
Hence, you are alive.
Arguments that are valid can be described as having:
False premises and a false conclusion.
False premises and a true conclusion .
True premises and a true conclusion.
The only combination of premises and a conclusion that a valid argument cannot have is true premises and