An Introduction To Logical Fallacies

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By aaron

What is a fallacy? Simply stated, a fallacy is a mistake in an argument that puts the conclusion of the argument into question. There are dozens of fallacies that have names, but all of them fall into one of two main groups.

Those in the first group are called formal fallacies. A formal fallacy is a mistake in the logical structure of an argument. Because the flaw lies in the logical form of the argument, an argument with a formal fallacy will necessarily be invalid. Here is an example:

If it rains, the streets will be wet. The streets are wet. Therefore, it rained.

This argument has a formal fallacy, because the logic is not valid. In other words, the conclusion will not necessarily be true, regardless of whether or not the premises are true. Even if it is true that rain makes the street wet, and even if it is true that the streets are wet now, it still doesn't necessarily mean it has rained. The logical structure of this argument doesn't account for the other reasons why the street might be wet, such as a water main break. Thus, this argument is not valid.

This is in contrast to an informal fallacy. With an informal fallacy, the argument might have a valid logical structure, but the conclusion is still not sound because one or more premises is untrue. Consider this example argument:

Mammals can't lay eggs. That animal just laid an egg. Therefore, that animal is not a mammal.

This argument has a valid logical structure, because if it is true that mammals can't lay eggs, and if it is true that the animal in question has laid an egg, then it is correct to conclude that the animal is not a mammal.

However, this argument, while valid, is not sound, because one of the premises is in fact false. There are actually two mammals that lay eggs, one called a platypus and another one called an echidna. In short, it is incorrect to claim that mammals can't lay eggs. Thus, you cannot conclude that the animal being observed is not a mammal just because it laid an egg. The animal could be platypus. Mistakes such as this are called informal fallacies.

I like this example because it demonstrates how difficult it can be to catch an informal fallacy. In many cases, such as this one, your ability to catch the fallacy might be based on subject knowledge that you may or may not have. For example, if you didn't know that there was a mammal that lays eggs, you likely would not have caught this fallacy.

It is important to note that the presence of an informal fallacy does not necessarily mean the conclusion is wrong. If it turns out that the animal really isn't a mammal, then even with a fallacy in our argument, the conclusion would still be correct. Perhaps the animal is a bird, which would also explain why it laid an egg. An informal fallacy like this simply means we need to strengthen our argument by either clarifying it or providing more evidence.

Those are the basic concepts of formal and informal fallacies. Starting with the next lecture, I'll go beyond the basics and start to explain some of the named fallacies that I mentioned earlier. Thank you for listening! #logic #skepticism #critical thinking #fallacy #fallacies

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