Deductive and Inductive Arguments Now that you’re familiar with the basic
structure of arguments
where there are always one or premises and a conclusion,
Let’s look more closely at different types of arguments.
Remember—making an argument is all about staking out a position
on a specific problem or issue. Now think about all the different types of
problems one could confront.
There are problems in everyday life: where should we eat dinner?
which house should we buy? Which school is better for my kids?
There are problems that college students like you deal with:
how many classes should I take next semester? What should I major in?
Should I get an internship or study abroad? There are problems in business:
do we need to hire another employee? What database software should we use?
How should we design this marketing campaign? There are political problems:
what's the best income tax structure? What should our energy policy be?
As people make arguments to resolve these
(and many more) issues, they may offer many different types of premises
in defense of their position. We're going to dig deeper into two kinds of
arguments:
deductive arguments and inductive arguments.
To get started consider the following two examples:
in each case there's a question or problem to be resolved,
and examples of possible premises that could be offered as part of an argument
to address that question.
In the first example, the problem is that you and some friends are trying to
decide where you should go for dinner
the Red Iguana or Cafe Trang. There are two premises:
1) Red Iguana is open until 10 p.m.;
Cafe Trang is only open 'til 9pm. And 2)
The Red Iguana has the best Mexican food in the area.
In another example, the problem is trying to answer the question,
how many classes should I take next semester? Again there are two premises:
1) Tuition for 12 credit hours will cost $3,100;
tuition for 15 credit hours will cost $3,700.
2) It's better to take more classes every semester
so that you can get done faster. In both examples,
the first promise offered was a FACT
that can be proven to be true or false. You can look up the hours for each
restaurant
or the tuition cost for different numbers of credit hours. However,
the second premise offered was an OPINION that cannot be proven
to be true or false. People may disagree on whether the Red Iguana has the
best Mexican food
or if it's the best idea to take a lot of classes to finish your degree more
quickly.
Facts and opinions have different relationships with the conclusions that
are drawn,
and result in different types of arguments. For example let's say it's
currently 9:05pm
when your friend asks, "Should we eat at the Red iguana or Cafe Trang?"
You reply, "Well Cafe Trang closed at 9pm
but the Red Iguana is open until 10, so we need to go to the Red Iguana."
If you're correct - if the facts are true
then your conclusion HAS to be TRUE. Your only option is Red Iguana.
An argument in which the conclusion necessarily follows from the premises
- if the premises are true then the conclusion is also true -
this is a DEDUCTIVE argument. On the other hand, what if it's only 8pm?
Both restaurants are still open and available. When your friend asks which
one you should choose,
you reply, "Well Red Iguana has the best Mexican food in the area.
Cafe Trang is okay, but it's not that great. We should go to the Red Iguana."
In this case the decision to go to the Red Iguana
cannot be said to be "true." The conclusion does not logically follow from the
premise.
Just because you think the red iguana has better food does not mean you
- or your friend -
must choose it. You've simply tried to build a case for why it's the better
option.
An argument in which acceptance of the conclusion
depends on the strength of the premises - in which the premises do not prove
but merely support the conclusion - is an INDUCTIVE argument.
Deductive Arguments. Remember, a deductive argument is one in which the conclusion
necessarily follows from the premises
- if the premises are true the conclusion is true. What if you don't like the
conclusion
- if say you really wanted to go to Cafe Trang?
Well, it's too bad the promises don't support that conclusion.
In our example, if the restaurant
is indeed closed, you're outta luck. The conclusion that you should go to the Red
Iguana, in some ways, states the obvious -
if you only have two options and one of those options is closed
(the premises of the argument), then it really goes without saying that you must
go to the other one.
It doesn't matter if you don't like the conclusion (say for example that you
really wanted to go to Cafe Trang instead) -
unless you can disprove one of the premises of the argument,
you have no choice but to accept the conclusion. Anytime you're dealing with
deductive arguments -
whether you're evaluating an argument someone else has made
or constructing your own argument, you shouldn't start with the conclusion.
When you evaluate someone else's argument, you must avoid
"jumping to" the conclusion right away and deciding,
without consideration of the argument as a whole, whether you agree with it or
not.
Similarly, when you make your own argument,
you must avoid picking out your conclusion ahead of time
and then finding ways to justify it. Instead,
you need to start by examining or uncovering the premises
(reasons and evidence), then follow where those premises lead -
what logical conclusions can you draw from the evidence?
So, when you evaluate a deductive argument,
you need to ask to questions:
1) are the premise is true? and
2) is the form of the argument valid? If the answer to both questions is YES,
then you have a sound argument. The first question is fairly simple,
although it does not mean that it will always be easy to answer.
Premises in deductive arguments are facts that can, at least in theory,
be proven true or false. They refer to condition or states of things -
is the restaurant closed, for example. In this case
it's pretty easy to find out if that premise is actually true or not -
you could drive to the restaurant and find out. The second question -
whether the argument is valid or not - refers to the logical structure.
An argument is valid if it's not possible for the premise to be true
AND the
conclusion to be false. In our example, the premise tells us that there are two
restaurants to choose from
and one of them is closed. The conclusion, then,
is that we must go to the other - open - restaurant.
If that premise is true, then it is not possible for the conclusion to be wrong -
it MUST be true as well. Inductive arguments.
As you heard, an inductive argument
is one in which the conclusion is supported (but not proven),
to a greater or lesser degree, by the premises. The conclusion goes beyond the
premises -
the conclusion that you should go to Red Iguana is not logically implied by the
statement -
that it has the best Mexican food. Maybe your friend doesn't really feel like
eating Mexican food;
in that case, he could offer a counter-argument with reasons why you
should go to Cafe Trang instead.
In the case of inductive arguments,
the evaluation process has to be different than deductive arguments.
You cannot necessarily "prove" or "disprove" the premises,
nor can you determine if the premises lead inevitably to the conclusion or not.
Instead you must ask the following questions:
1) are the premises true or at least acceptable?
2) are they relevant to the issue at hand?
and, 3) are the premises compelling enough
(students of critical thinking call the sufficient) to justify the conclusion?
Your answers to these three questions will help you evaluate how STRONG
or WEAK the argument is. So let's take a closer look at these questions.
First, with inductive arguments
you may find premises that are not easily assessed as true or false;
rather than facts, you will often have matters of opinion.
The assertion that the Red Iguana has the best Mexican food is a matter of
opinion
over which people may disagree. In evaluating this argument,
you need to consider whether this premise is ACCEPTABLE -
can you accept it as reasonable? In this case,
you might look at restaurant reviews or publish lists of the "Best of Salt Lake City."
You might poll your friends to see how many people agree
with this opinion. Second, you need to decide whether that premise -
in this case the opinion that the Red Iguana has the best Mexican food -
is relevant that is is the premise related to the issue at hand?
Here, it does seem relevant to consider the reputation of a restaurant
when deciding where to eat. However if it's said that we should go to the Red Iguana
because it has the nicest parking lot,
you might question whether that reason is really relevant to deciding where to eat.
Finally, you need to consider whether the premise is sufficient to justify the
conclusion -
is the opinion that the Red Iguana has the best Mexican food
really enough to base a decision on? Are there other things you might want to
consider -
for example, how long is the wait for a table?
How good is the service? Evaluation of inductive arguments falls into a range
from weak(er)
to strong(er). If you determine that the premise is highly acceptable,
is relevant to the issue at han, and is enough of a reason to base your decision on,
then you would conclude that the argument is fairly strong.
If you find that the promise is highly acceptable
and it is relevant, but it's not really sufficient enough to make a decision
based on that premise alone,
then you might say the argument is "so-so" - not weak,
but not strong enough. You can improve the argument by offering more promises
to support it -
"Yeah, the food great, and so is the service. Plus I just called and they said
we could get a table in 10 minutes."
Or alternatively, you might offer a counter-argument =
"Sure, the food is good, but their service is awful and there's always a really long wait."
As you may have figured out, inductive argument can take on many different
forms. There are:
Generalizations are where arguments involve making a general claim
based on limited or specific evidence (for example drawing conclusions about
the opinion of the nation as a whole
based on public opinion polls of some smaller number of people.)
Analogies are similar to generalizations and that they involve proposed
similarities;
when making an analogy, you draw conclusions about one situation
based on what you know about another -
allegedly similar - situation. General principles are the opposite of
generalizations.
These arguments involve applying general principles (for example,
qualities a group of people are assumed to have) to a specific case
(individual member of that group.) Causal Reasoning arguments offer what is
determined to be the best possible explanation for why something has
happened; they offer an argument that one thing
necessarily lead to another happening. Now you should have some idea
what deductive and inductive arguments are, and how they differ.
Many of the topics introduced here will be discussed in more detail later.
You will learn more about how to evaluate the truth and acceptability a
premises,
as well as how to assess the relevance and sufficiency of those premises.
You will also learn how to use causal reasoning
to both develop and assess arguments.