Valid, Sound, Strong, Cogent 2018.docx

# Valid, Sound, Strong, Cogent 2018.docx - Valid Sound Strong...

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Valid, Sound, Strong, Cogent page 1 As we have already discussed, the difference between a deductive argument and a non- deductive argument has to do with the intended relationship between the premises and the conclusion. A deductive argument intends for the premises to guarantee the conclusion. A non- deductive argument intends for the premises to make the conclusion likely, not guarantee it. As we saw in the lesson on Deductive vs Non-Deductive, when the premises of a deductive argument do actually succeed at guaranteeing the conclusion, the argument is said to be valid . If the premises of a deductive argument don’t succeed at guaranteeing the conclusion, the argument is said to be invalid or not valid. In logic, the terms ‘valid’ and ‘invalid’ are technical terms. They apply just to the relationship between the premises and the conclusion, not to the question whether the premises are true. If a deductive argument is valid, then the premises are properly related to the conclusion. But it doesn’t follow that a valid deductive argument is good. In order for a deductive argument to be good, the premises have to be true as well. We will now introduce a new term. If you have a deductive argument that has all true premises and is also valid, then the argument is said to be ‘ sound ’. That means that if you have a deductive argument that is invalid or has at least one false premise, the argument is said to be ‘ unsound ’. (You may now wonder if all sound arguments have true conclusions. Yes, they do. A sound argument has true premises, and because it is valid, the premises cannot be true while the conclusion is false; so that means it has a true conclusion.) Here is an example of a sound deductive argument: All humans are mortal. The President of the United States is a human. So the President of the United States is mortal. This deductive argument is sound, because both its premises are true and the argument is valid. Here is an example of an valid deductive argument that is unsound. 1. All humans are fast swimmers. 2. My uncle is a human. So, my uncle is a fast swimmer. This deductive argument is unsound because the first premise is false. Not all humans are fast swimmers. Although the argument is unsound, it’s still a valid argument, because the premises do guarantee the truth of the conclusion. In other words, if the premises were true, then the conclusion would also have to be true. Remember that validity only applies to the relationship of the premises to the conclusion; it does not require that the premises be true. Here is an example of an invalid deductive argument (and that makes it unsound too). All dogs are mammals. Monty is a mammal. So Monty is a dog. This deductive argument is unsound because it’s invalid. It’s logically possible that even if the premises were true, the conclusion could be false. Yes, all dogs are mammals. But that does

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Valid, Sound, Strong, Cogent page 2 not entail that all mammals are dogs! There are many non-canine mammals. Monty could be a cat (or a human) and the premises would still be true. But the conclusion would be false.
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