Logic

Why do we attempt to avoid fallacies?

Comments

Fallacies are forms of reasoning that fail to provide support for the conclusions reached via that reasoning. In other words, the premises could all be true, but the conclusion still false. Just because something is a fallacy does not make the conclusion of such reasoning false, however.

For example, here is a (deductive, logical) fallacy with a true conclusion:

If my name is Nicholas D. Smith, then I have a very common last name.
I do have a very common last name.
Hence, my name is Nicholas D. Smith.

All the premises are true, and so is the conclusion, but the reasoning is fallacious (called "affirming the consequent"), because the truth of the premises does not in any way support or ensure the truth of the conclusion. To see this, consider another example of the same sort of inference (affirming the consequent):

If I am swimming, then I am wet.
I am wet.
Hence, I am swimming.

Nah! I live in Portland, Oregon--folks here are wet most of the year from the rain. So even if the first two premises of the little argument above are true, notice that it doesn't "follow" that the conclusion is true. Hence, the truth-value of the conclusion is in no way supported or assured by the premises. That's what it means for something to be a fallacy.

To answer your question now, we attempt to avoid fallacies because we care about what is true and we want to believe what is true and not what is false (at least when we are being reasonable). So we want to avoid reasoning that does not help us (and may actually hinder us) from our pursuit of truth.

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