Now that you can identify premises
and conclusions, how are they structured into arguments?
Deductive and Inductive Arguments
Deductive arguments are arguments that show a tight
connection between the premises and the conclusions. There is
no possible way the conclusion could fail to be true if the premises
are true. (That is not to say, of course, that the premises are
true.) Arguments in mathematics and in pure logic are often of
this sort: "If no one watered my plants during my vacation,
they will all die. No one watered my plants during my vacation.
Therefore, my plants have all died." The premises of that
argument might possibly be false. But, if they are both true,
then there is no way the conclusion can be false.
There
are other arguments with a looser connection. If the premises
are true, then the conclusion is likely to be true also; it
would be surprising if the conclusion were false; we have
good reason to think that the conclusion is true, and so
on. These are the inductive arguments.
The AIDS argument above is inductive. The conclusion of the argument
is that it seems probable that in some way the disease
has been successfully stalled.
In
both sorts of arguments, the premises support the conclusion
if those premises are true. But if they are false, they
provide no such support. Discovering that a premise is false,
then, undercuts the force of both deductive and inductive arguments.
Because
inductive arguments are not conclusive, they may be weakened
(perhaps rejected entirely) even if we continue to recognize
that their premises are perfectly true. If, for example, researchers
were to identify a virus very similar to the AIDS virus, and
if they discovered that the incubation period for this virus
in chimpanzees often exceeded fifteen years, that discovery would
seriously weaken the argument in the example. But it would not
show that any of the premises of the original argument were false.
It would not show that some people have not survived, and in
good health, for twelve years after having been infected. It
weakens the argument by introducing another possibility, that
the AIDS virus may have a longer incubation period than previously
thought plausible.
Inductive
arguments can also be strengthened by the introduction of new
data. If it were discovered that there is a similar virus which
infects chimpanzees, and for which many chimpanzees develop a
resistance which enables them to live out a normal life span
in good health, that would further bolster the conclusion that
perhaps some humans can make a similar defense against AIDS.
w C. Putting it into
your own words
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