1996-01
[this is a typed copy of the handouts printed by instructor Kurt Roggli]
When one offers an argument, one is putting forth something which,
as an objective matter, is either good or not; that is, there are objective
criteria for the goodness of an argument. There is, in principle,
a way of determining the worthiness of an argument which is independent
of any one personfs feelings about its worthiness. An argument stands
or falls on its own merits. Its worthiness is not determined by simple
subjective considerations such as the character, nature, or motives of
the person offering it. Charles Manson might present a beautiful
argument; Mother Teresa might offer a terrible argument.
Notice the parallel here with statements. The truth or
falsity of a statement about the world is independent of the character
or motive of the person who asserts it. Whether a statement about
the world is true or false is determined by the way the world is.
2. Kinds of Arguments
There are basically two kinds of arguments: deductive arguments
and inductive arguments. These kinds of arguments are distinguished
according to the way in which the premises allegedly support the conclusion.
2.1 Deductive Arguments
A DEDUCTIVE argument is
An argument in which the premises are claimed to provide necessary
support for the conclusion.
In other words, the premises are claimed to support the conclusion
in such a way that if the premises are assumed true, it is impossible for
the conclusion to be false.
Here is an example of a deductive argument:
(1) All tigers are animals. [premise]
(2) Tony is a tiger. [premise]
Therefore, (3) Tony is an animal. [conclusion]
Note that if we assume that (1) and (2) are true, then it must be the case that (3) is true.
2.2. Inductive Arguments
An INDUCTIVE argument is
An argument in which the premises are claimed to provide only probable
support for the conclusion.
That is, the premises are claimed to support the conclusion in
such a way that if the premises are assumed true, then, based on that assumption,
it is only probable that the conclusion is true.
Here is an example of an inductive argument:
(1) Most Republicans are rich. [premise]
(2) Barbara is a Republican. [premise]
Therefore, (3) Barbara is rich. [conclusion]
Note that, in this case, if we assume that (1) and (2) are true, then (3) is very likely true.
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3.1 Validity and Invalidity
A deductive argument, remember, is an argument in which it is
claimed that the conclusion follows necessarily. If the conclusion
really does follow necessarily, then that deductive argument satisfies
condition (1) mentioned above and is said to be VALID. Any deductive
argument which fails to satisfy condition (1) is said to be INVALID.
3.2 Strength and Weakness
An inductive argument is an argument in which it is claimed that
the conclusion follows probably. If the conclusion really does follow
probably, then that inductive argument satisfied the first condition and
is said to be STRONG. Any inductive argument which fails to satisfy
the first condition is said to be WEAK.
3.3 An Informal Strategy
It is often a difficult and complex matter to determine the validity
or strength of arguments. A good course in logic or critical reasoning
is very helpful because in it one learns techniques for determining validity
or strength.
Lacking these techniques, one might adopt the following strategy.
In the case of deductive arguments, ask yourself whether you could imagine
the premises true and the conclusion COGENT. Any inductive argument
which fails to satisfy both condition is said to be UNCOGENT.
In general, the degree of confidence one has in the truth of
the conclusion of a valid or strong argument is proportionate to the degree
of confidence one has in the truth of the premises. The greater onefs
confidence that the premises of an argument (which is valid or strong)
are true, the greater onefs confidence that the conclusion is also true.