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Professor Doug, you said you would tell me more about something you called logical fallacies.
Yes indeed. You remember how we talked about the difference between formal and informal
reasoning. I do, you told me that formal reasoning was
deductive while informal reasoning was inductive. If I applied deductive reasoning correctly
true premises guaranteed me a true conclusion, but I am not sure why.
That is because formal reasoning is all about the pattern or organization of the ideas we
work with. Because we concentrate on the pattern instead oif the content of these ideas we
typically work with letters or other symbols instead of the actual words. The main types
of deductive logic you study in school will either be what we call syllogisms dating back
to ancient Greeks or the propositional logic developed at the turn of the twentieth century.
Formal fallacies are mistakes we make in working with these patterns.
Would you give me some examples? Yes. If I say all fruits are nutritious and
apples are fruit, what would follow? That's easy. Apples are nutritious.
Correct. Now what if I said all Republicans are conservative and then that the senators
from Idaho are conservative? I guess it would follow they must be Republicans.
Sorry, this is incorrect. You see, it may well be true in itself, but there is still
the possibility that we have conservative senators who are Democrats or Independents.
That is why this second pattern is an example of a mistake or fallacy. We even have a name
for it, but I will not go into that now. Oh oh, maybe formal logic is not that easy
after all. I would have to learn more about how to test to see if we are using the right
pattern. Exactly. Now that was an example of a formal
fallacy. With informal fallacies we look at things differently. Since here we are working
with facts of one sort or another we need to know that the information we start with
is actually relevant to what we intend to prove and also that it sufficiently rules
out alternate interpretations. Again, may I have some examples?
Yes. One of the most common informal fallacies occurs when we say that something must be
a good product because a famous celebrity has endorsed it. Another is to say I must
be right because you cannot prove me wrong. Actually we could list about a dozen or so
types of things that might be mistakes in our reasoning. What is tricky, of course,
is that the line between something acceptable and something fallacious may not always be
that clear. As a result students often get carried away and see everything as a fallacy.
So again I would need to take a lot more time to get a feeling for when something works
and when it doesn't. I am afraid so. A quick rule for both formal
and informal reasoning is to think in terms of counterexamples. See if you can tell a
story with the same information in the premises but add something more so that we have an
opposite conclusion. That is what I did in talking about Republicans and conservatives.
If it even makes sense, you know that you do not have a good deductive pattern. If it
makes sense but would be highly unlikely then you might still have a case that is what we
call inductively strong. So just because an argument fails the test
for deductive reasoning does not mean it is a bad case?
Yes. A deductive argument might be valid or invalid. If it is valid and the premises are
true we call it a sound argument. An inductive argument by definition is never valid but
it can be either strong or weak. If it is strong and again the premises are true we
call it cogent. But that should be enough for now.