An argument has good logic
when its premises support its conclusion. When an argument has good logic, we
say its conclusion follows from its premises.
Bad or incorrect logic
when the premises fail to support the conclusion, or the conclusion
does not follow from the premises.
It is extremely important
that you understand exactly what
we mean by “support.” We said an argument has good logic when its premises support
its conclusion. By this we mean either:
the premises if true guarantee the truth of the conclusion.
if true make the conclusion likely.
arguer claims support
in the first way above, she is said to be arguing deductively. That is,
when an arguer argues deductively, she is claiming that the premises guarantee the truth of the conclusion, i.e., the conclusion can’t
be false if the premises are true.
Typical kinds of deductive arguments include
Argument based on mathematics
Argument from definition
Hypothetical syllogism (argument containing one or more conditional premises)
If an arguer claims support
in the second way above, she is said to be arguing non-deductively, or
inductively. That is, when an arguer argues inductively, she is claiming
that the premises make the conclusion likely, i.e., conclusion might be false if the premises are true, but it’s not likely
to be false if these premises are true.
Typical kinds of inductive arguments include
Argument from analogy
Generalization from a sample
Argument from authority
Argument based on signs
So, deductive arguments
good logic are those in which the conclusion must be true if the premises are
true. We call deductive arguments with good logic valid arguments.
By contrast, deductive
with bad logic are those in which the arguer claims the premises guarantee
the conclusion, but the arguer is mistaken: it is possible after all for the
premises to be true and the conclusion false. Such arguments are called
Validity (correct deductive
logic) depends entirely on logical
form. See notes on "Refutation by Logical
Inductive arguments with
good logic are those in which the
premises make the conclusion likely. Such arguments are usually called strong
Inductive arguments with bad logic are called weak. Weak arguments are
those in which the arguer claims the premises make the conclusion likely, but
the arguer is mistaken: the conclusion isn’t really likely.
Notice that in all this
discussion of good and bad logic,
we have said NOTHING about whether the premises are in fact true. That’s
an argument can have good logic, strictly speaking, even if its premises
are false. Now, I am not saying you should accept arguments with false premises.
Such arguments are defective, of course, and should be rejected. You want
to have both good logic and true premises. All I’m saying is that good
logic and true premises are two different things. An argument can have good
logic – its premises can support its conclusion in the sense of “support” described
above – even if its premises are in fact false.
Here’s an example.
All bears fly.
Lassie is a bear,
Therefore, Lassie flies.
Do you see how this
argument is logically correct, in the
sense that its premises if true support its conclusion? I.e., its conclusion
must be true if its premises are true? I.e., it follows from these premises
that Lassie flies? The premises of this argument are not true, but the conclusion
would have to be true if the premises were true.
An argument like this is
logically correct but factually
incorrect. It passes step 3 of the Critical Thinking
Checklist, but fails Step
4. You’d certainly reject it – but not because it has any logical error. Its
logic is perfect; what’s wrong here is that the premises are false.
If a deductive argument is valid AND has all true premises, the argument is said to be sound.
If an inductive argument is strong AND has all true premises, the argument is said to be cogent.
Typical logical mistakes are
occur only in deductive arguments,
since validity is a matter of form. (See notes on Refutation
by Logical Analogy.)
Formal fallacies result from mistaking an invalid form for a valid one, because
of a resemblance between the valid and invalid forms.
comprise all the other common
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