Lecture Five: Assessing Deductive Arguments
October 5, 2011
Identifying Deductive Arguments
Deductive Arguments are one of the following:
- The conclusion must be true, if the premises are true.
- The conclusion cannot be true, even if the premises are true
Sound Deductive Argument
- Empirically true or necessarily true
- Conclusion must be true because the premises are true.
P1. 2+2 = 1+1+1+1
P2. 4 = 1+1+1+1
P3. 1+1+1+1 = 1+1+1+1
C. 2+2 = 4
Valid Deductive Argument
P1. All swans are birds
P2. The Swan of Tuonela is a swan
C. The Swan of Tuonela is a swan (valid deductive argument because we do not
know the truth of premise 2, we must make assumptions)
Necessary and Sufficient Conditions
In a conditional claim, the antecedent is a sufficient condition for the consequence.
However, the consequent is only a necessary, not sufficient, condition for the
If food if delicious, then it makes my mouth water
= If the food is delicious, then that is sufficient to make my mouth water Only if my mouth waters then food is delicious
= It is necessary for my mouth to water in order for food to be considered delicious
Stipulative Definition: A necessary condition is a condition which must be satisfied if
something else to occur while a sufficient condition is a condition which, by itself,
ensures that something else will occur.
Supplying Missing/Assumed Premises in Deductive Arguments
Terms are subjects or predicates of claims.
1. Number each term in the given claims
2. Some terms in the argument will be r