January 23, 2014
Two considerations that are relevant to the structure of the arguments:
1. Whether the premises are meant to support the conclusion when taken together
2. Whether the premises support the conclusion separately (contains lemmas)
Degree of support for conclusion
Two broad types of reasoning: (pertaining to degree of support)
Deductive arguments vs. inductive arguments (nondeductive)
Deductive argument: degree of support is meant to be extremely high
If the premises are true the conclusion is guaranteed to be true
Deductive argument: an argument the truth of whose premises is intended to guarantee
the truth of its conclusion; i.e. an argument intended to be such that if the premises are
true, the conclusion must also be true.
Inductive/Nondeductive argument: an argument the truth of whose premises is
intended not to guarantee, but to make likely, the truth of its conclusion; i.e. an argument
intended to be such that if its premises are true, its conclusion is also probably true.
Ex. of a deductive argument:
If Descartes is correct, the mind and brain are tow distinct entities. If the mind and brain
are two distinct, life after death is possible. Therefore, if Descartes is correct, life after
death is possible.
If P, then Q. (P▯Q)
If Q, then R. (Q▯R)
If P, then R. (P▯Q)
^ Hypothetical Syllogism
All As are Bs.
X is B.
\ X is A. } ▯deductive
The vast majority of social conservatives subscribe to some organized religion or other.
Since the author is clearly a social conservative, he subscribes to an organized religion. Some As are B.
\ X is B. } ▯Inductive
She won’t be going to the party. After all, she’s been working the late shift almost every
night the past couple of months. Also, she just doesn’t really like those types of parties.
If one premise is deductive and another is inductive, treat the whole argument as
Lecture 4Assessing arguments: Soundness, premise truth, and
logical strength Assessing premise truth
• Untrustworthy and trustworthy sources
• Conflicting and confirming evidence
• Conditional premises, biconditional premises, definitions,
and the method of counterexamples
Once we’ve identified an argument by clarifying its elements and
structure, we are in a